Just now my MacBook refused to wake up from sleep. All I got when I lifted the lid was a black screen and the spinning color wheel (usually the password prompt appears).
Panicking that I might have lost data, especially considering I have FileVault enabled, I closed the notebook, let the computer go to sleep again, and lifted the lid for a second time. Hey presto -- the MacBook woke up this time around and I was able to enter my password.
So one possible solution to a slumbering MacBook (or other laptop) that refuses to wake is to hit the virtual snooze button and wake it up again a few seconds later.
[robg adds: I thought we had this potentially-frozen-laptop fix elsewhere in the database, but a few searches this morning came up blank. I've used the same technique myself on a G4 PowerBook a few times, though my MBP has been (thankfully) free of the same issue.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071001103919546