Before I say anything else, let me say this: odds are, your spring-flowering bulbs will be fine, despite what nasty surprises Mother Nature decides to throw at them. Every time I experience a little anxiety over the welfare of my beloved bulbs, I remind myself that many of these daffodils were already here when we bought the house, and have been blooming reliably for at least 20 years. I've added hundreds of additional bulbs since we purchased our home, including hyacinths, muscari by the truckload, scilla, tulips, daffodils, and lilies. I have yet to experience a weather event that significantly damaged the blooms of my bulbs, though there have been years with shorter displays than others. That said, let's examine a few factors that may influence the impact a late winter freeze may have on your little beauties!
The first factor to consider is what kind of spring-flowering bulbs you have. There are some very early blooming spring bulbs that seem impervious to cold and snow. I've had crocuses push their way up through the snow. Snowdrops, hyacinths, and some daffodils also seem to be particularly resistant to the cold, wet spring precipitation. An extended freeze, with daytime temperatures that remain below 29 degrees, might cause some damage, but an overnight frost will generally not cause any lasting damage to these tough little bulbs.
Tulips and lilies are a little more prone to damage once they've formed buds. If a hard freeze or an extended period of cold is forecast, it would be wise to cover your tulips and lilies to ensure you get to enjoy their blooms this year. If you are taken by surprise, and your tulips and lilies are damaged, don't despair. You will likely lose this year's blooms, but they will continue to store energy in the bulb to return to their regular blooming cycle next year.
Probably the number one deciding factor in whether you need to offer protection to your spring bulbs is what type of cold weather you will be having. If the temperatures are dropping below freezing overnight, and a frost is predicted, you probably don't need to worry, as long as the daytime temperatures will rise above freezing. Most spring bulbs won't be phased at all by short periods of cold and frost. Likewise, a late snow isn't likely to cause damage, as snow actually acts as an insulating blanket, protecting the foliage and buds from extreme fluctuations in temperature. I've actually directed my husband and kids to shovel the snow from the sidewalk onto my flower beds when the bulb foliage is poking through, to add extra moisture and insulation from the cold.
You do need to be concerned, however, if you have a period of extended cold temperatures, with daytime temperatures staying below 29 degrees, or a sudden drop into the low 20's and teens. This is particularly worrisome if the cold temperatures are accompanied by cold winds, which have a drying effect on your plants. The combination of wind and cold isn't any kinder to the leaves of your plants than it is to your skin and hair, often drawing moisture out and leaving the leaves limp and damaged. This is sometimes referred to as dry wilt, as the plant loses moisture faster than it is able to replace it.
Some gardeners employ a tactic used by the Florida citrus growers, particularly if the plant is already in bloom: they spray their plants with water to form a layer of protective ice, to prevent them from losing too much moisture to the winds and dry, cold air. The ice also prevents injury by helping the plant maintain a constant temperature, and protecting it from excessive moisture loss. This is a technique used by the Parks Department in Holland, Michigan, when the tulips for their annual Tulip Time Festival are endangered by prolonged low temperatures. 
Another key consideration is how far along your bulbs are in their emergence and bloom cycle. If the foliage emerged fairly recently, you probably don't have much to worry about. Foliage is not often damaged by cold. If it is, the plant still often recovers fully and goes on to bloom without any adverse affects.
Buds and blooms, however are more susceptible to the cold. The closer they are to blooming, the more prone they are to damage by frigid temperatures. If you have tulips or other bulbs in bloom, it might be wise to cover them, or cut the blooms to enjoy indoors in a vase. The flowers may appear to have withstood the cold without damage, but as the flower stalk thaws, it may become soft and droopy, and be unable to support the weight of the bloom.
The location of the bulbs on your property may also have an impact on how much they are affected by the cold. Plants located close to the house may fare better, as they have some protection from the wind, and may benefit from some heat radiating from the house at night. This is a double-edged sword, however. Flower beds with southern exposure and in warmer microclimates created by proximity to your house tend to sprout first, as the soil warms there first, so they are most in danger of frost damage. Low-lying areas, where the lowest temperatures tend to pool, are at more risk than plants on a hillside or slope. Areas that have experienced a very dry winter are also more prone to damage due to the cold, as the plant is at risk for dehydration. Some experts recommend a deep watering if a hard frost is forecast, both to keep the soil temperatures elevated, and to prevent over-drying of foliage.
One of the best preventative measures you can employ, well in advance, is to mulch the beds in late fall, after the ground is frozen. If you mulch while the ground is still warm, it may keep the beds from freezing when the rest of the soil freezes, and encourage the bulbs to sprout too early. Mulching after the soil freezes helps maintain a constant soil temperature, and reduce frequent freezing and thawing as temperatures fluctuate. This also has the added benefit of reducing bulb "heaving," where the freezing and thawing cycles cause the bulb to come to the surface, where they are not protected by the insulating soil, and it helps the soil to retain moisture, which is critical for healthy bulb development in the spring. Almost any organic matter, applied 4-6 inches deep, makes an effective mulch: wood chips, straw, compost, mulched leaves (I run over them with my mulching lawn mower, which also conveniently bags them for me. I then use the mulched leaves as a protective covering in my flower beds, or add it to my compost bins), or pine needles and branches. You do want to avoid mulches that form a thick, impervious mat, such as grass clippings, as they won't allow water to penetrate to the soil, and may encourage the growth of fungal diseases.
If you did not mulch in the fall, and have tulips and other bulbs emerging and setting buds, it is not too late to add some protection if an extreme freeze is predicted. A loose, light mulch, such as straw, can be added around the tulip plants and buds, covering them by at least an inch, to protect them from extreme temperatures and hard frosts. This can be labor-intensive if you have extensive beds, as the mulch has to be removed once temperatures warm up, but it can definitely preserve your blooms! Just be sure to use "clean" straw without weeds or seeds, or your bulbs will be competing with grass and weeds for water and nutrients when the seeds sprout.
If you determine that you do need to cover your plants when extreme conditions are forecast, there are several factors to consider. You probably have several appropriate materials in your linen closet! Sheets work well as protective covers, as do lightweight blankets. If you have burlap or fabric landscaping cloth, those are also excellent options. The key is to avoid plastic or materials that don't retain heat, and may cause moisture retention that may damage your plants. If your plants are already in bloom, you will need to put garden stakes or some kind of supports to hold the covers up, and prevent the weight of them from breaking bloom stalks, which are less flexible than foliage. Heavy snow on top of sheets or fabric can also weigh down plants, so providing support can be crucial. I've gone so far as to set up folding chairs and tomato cages over my plants and draping sheets over them, to provide support and protection from extreme hard frosts. If harsh winds are also forecast, be sure to hold the edges of the cover down to the soil with rocks, bricks, or earth staples, for two reasons. First, your cover will do no good if it is blowing down the block. Secondly, shelter from drying winds may be as critical as shielding from frost or ice build-up, and wind has a way of finding its way under the unsecured edges of sheets and other covers.
Extreme cold over a period of several days can be especially damaging, but there are some ways you can add a few critical degrees of warmth under your covers. If you have strings of Christmas tree lights that are rated for outdoor use, you can drape those over and around your plants, to raise the temperature slightly. Even a couple of degrees can be the deciding factor between severe damage and slight browning of foliage. A decidedly lower-tech option is to place milk jugs or 2 liter soda bottles of water under the covers. They will absorb solar energy during the day, and radiate it back to your plants at night when the temperatures are lower. Bricks and cement blocks can be used the same way.
If frost damage does occur, don't cut off foliage, even if it is damaged, as bulbs need the foliage to feed the bulb. If it is still early in the season, and the plant sends up new, green foliage, then it is safe to remove the damaged leaves. However, if the foliage is in full swing, and it turns yellowish or soft, hold off on removing it until you are certain it won't recover. This year's foliage will store up food for next year's blooms! As a general rule of thumb, you should not cut back any foliage on bulbs until it has turned brown and died back, or your bulbs may not store up enough carbohydrates to last through the next winter and bloom season.
For a detailed explanation of Frosts, Freezes, and Microclimates, check out this excellent article by Dave's Garden writer Geoff Stein (palmbob).
Thumbnail image of hyacinth bloom in the snow: Flickr Creative Commons, by metrognome0, some rights reserved.
Crocuses in snow: Flickr Creative Commons, by Men In Black, some rights reserved.
Tulip foliage in snow: Flickr Creative Commons, by staticgirl, some rights reserved.
Hyacinth bud and foliage in snow: Flickr Creative Commons, by derekGavey, some rights reserved.
Siberian Squill and Daffodils in snow: Flickr Creataive Commons, by mccormicka, some rights reserved.
Snow on Kentucky Daffodils, by my husband, David Carson. All rights reserved.