Man, so many people losing the rubber feet and asking about how to put it back on... Use some common sense and think for yourself, eh? PB 12" is a computer, yes, but the rubber feet is just that: a piece of rubber glued on a plastic mold on an Al case.
I used Superglue. Now it won't come off again.
Superglue has nice property that it works best if you apply a very thin amount. So if you are worried about ruining your PB (I can't see how, but anyway...) apply a very thin layer, align the notch and there you go.