I used to have an interest in this topic as a super eco-friendly person. I had actually forgot about it until I saw a video on WalletPop (with my fave columnist Bargain Babe).
Allison Burtch has a great blog of what is actually being thrown away each night from grocery stores in her area. I have to say, I'm in agreement with her - that has to stop. Why on earth can there not be a "Good Samaritan" law to protect retailers from lawsuits from giving away their food? Expiration dates are set as a precaution - there's tons of food out there that you can eat way beyond the expiration date. Sometimes they're set as a marketing gimmick, too - if I convince you that your food is bad by X date, I'll get you to buy more of it at X date + one day.
Check out this find from one night in a dumpster!