I have lost all 4 feet on my 12" powerbook. What sucks is that the shell now sits directly on my desk, and it's scratching up from dust and crumbly stuff beneath.
Apple refused to do anything except offer me the 200-or-so-dollar standard repair (cosmetic damage is not covered under warranty). You can get rubber feet via resellers, but they're expensive ($9 or so for 5 feet), and you can only get the feet for the 15" tiBook, which are NOT the same.
So, just get yourself some nice little silicone feet for a couple of cents apiece and stick'em under there.
-s*