Same here
3 of 4 feet fell off over the past few weeks. Took it into an Apple store & they said they had to replace the whole bottom panel. Not the genius (was busy) but a floor person, so it could be wrong.
Seems like quite a few people have had this problem. Pretty stupid design too -- not only do they fall off, but it leaves an open hole to the inside of the machine.
I imagine if we bug the geniuses enough they'll replace them free -- like the white pre-unibody macbook case that kept cracking where you rest your hands -- Apple replaced that one for me three times!
btw, the link robbieduncan provided seems to be for a 15" MBP. Mine's a 13. Any idea if they're interchangeable? $15 isn't much and I'd be willing to pay it, but these really should go for about $1 for four.