As far as I know, that rubber bottom on the new white MacBook is attached to a panel, which keeps it rigid. Otherwise, that rubber would just flop around, obviously. I have 2 suggestions for you:
1) If you are close to an Apple Store, or are willing to drive to one, they will fix this problem for you in no time if you're under warranty. Often, simple non-user-caused flaws will even be taken care of outside of warranty, because they're usually just that nice.
2) If for whatever reason you can't make it to an Apple Store, or if you just want to do it yourself, I would take off the bottom panel that the rubber is attached to, and just try to re-glue it - using a good glue so it doesn't happen again. If you use super glue, do NOT be liberal - be mildly sparse. That stuff has ill-effects on some plastics.