I saw the post about repelling codling moths and I thought I would pass this on:
When I took my Master Gardener classes, I asked Colorado's bug guru (Dr. Cranshaw) about companion planting to repel pest insects. Here is what I learned: Insects have a very limited sense of smell, they can smell the opposite sex and their host plant/food, but that is about all. But they can smell those specific things for miles, in the case of beetles that fly in annually from farther south, maybe for as much as a hundred miles downwind. So basically we cannot plant to repel, only to attract. He said that mixed companion plantings are still a good idea because it creates a healthier more diverse enviroment than a monoculture, and is more likely to attract and support beneficial insects and organisms. We can also plant "trap" crops to attract pests away from the plants we are trying to protect to something they like better that we can spare.
For myself, this coming year I am going to get serious about crop rotation in the vegetable garden, and include annual plants to attract beneficials with my annual vegetables - but I'm going to keep the perennial herbs that supposedly repel pests in the perennial flower bed. They have probably been "repelling" by attracting beneficials, but mixing annuals and perennials is more trouble than it is worth.