This seems to be a common problem. Mine just fell off last night - the one below the Magsafe connection. Must get too hot there and the glue degrades. I didn't think this was an issue until recently when I started to feel it getting loose. I brought it up with my friend who has the same unibody and he said that his already fell off and he just superglued it back on. From the sounds of it, he hasn't had any problems since.
I've been reading about it and it looks like you can be replacement feet in sets online, possibly even from Apple themselves. This option would probably be done if you don't have Applecare or warrantee anymore. Otherwise, you should just take it in and they'll fix it for you. I talked with the AppleCare guys and they said their not sure what they tend to do with this, but they have heard of them just changing out the entire bottom panel. Seems kind of overkill for such a small piece, but whatever...let AppleCare pay for it. The caveat is that you won't have your SN laser etched on the bottom anymore. Not a huge deal. From what I've read online, the bottom panels run around $50 if you don't have AppleCare, so that might be a good compromise if you have serious damage to the bottom and/or missing the rubber feet.
Anyway, kind of annoying, but it's common in laptops. The glue just doesn't last that long. It happened in my old Macbook Pro and I'm sure it'll happen in future ones.