I have Johnny's Selected Seeds book called "Growing Garden Seeds". In it, it lists approximately how long a seed is good for both with no specific storage conditions and Extended storage where you put the seeds in a freezer. With tender plants like tomatoes - is it safe to freeze the seeds? I know this is a silly question, but I don't want to lose my heirlooms by trying this.
For starters, most viability charts are based on storing in cool, dry conditions. If you can do that, you can keep seed surprisingly long times. Keep in mind that the viability charts are based on 50% viability after whatever the listed period, and that you will often get germination far longer than that.
Professional freezing is how seed banks keep seed. This is done, of course, in liquid nitrogen. Such seed is considered good for at least 40 years.
Home freezing presents some problems. Seed should be stored in a 0 degree freezer. Most upright freezers, and the freezer compartments in fridges, do not go that low. The problem is, if the seed isn't fully dry (to 5% moisture), and it gets frozen slowly, large ice crystals form and puncture the cell walls, thus killing the seed. The faster you freeze the seed, the less likely this is to happen.
In addition, you should never store seed in a self-defrosting freezer, because it is then subject to a possible freeze/thaw/freeze sequence.
When removing seed from the freezer, it should remain in its wrapper (most of us use plastic bags or canning jars) until reaching room temperature. Otherwise condensaton can form, and this moisture can negatively effect germination rates.
I keep seed in my spare refrigerator, in the summer, and the cool garage in the winter. The reg. night temps dont get below 30 in the garage. They usually are viable about 4 years. I wouldnt freeze them.
I have a box type freezer. Seeds have beed stored in it for the past 5 years now. I have noticed no excessive loss of viability. This summer I'm growing some 5 year old tomato seeds in fact for fresh seed. 80% viability was average. The peas that I'm growing are 3 year old seed. No apparent problem there either.
I think part of the problem with seed storage (and freezing)is as Brook mentioned having thoroughly dry seed to start with. In the desert it isn't as difficult to achieve this as in some parts of the country perhaps.
I would concur that the proper preparation, packaging and handling of the seeds is important to successful long-term storage at sub-freezing termperatures. The trick is to slow the seed's metabolism and keep the conditions stable without damaging them at the cellular level. Obviously, not everyone can afford sub-zero storage devices (perhaps with the exception of large companies and large government programs).
Here is a link to a seedbank program in Sweden that you may find of interest.