My Powerbook G4 sometimes will not wake up from sleeping -- I hit all the keys and nothing wakes it. I found that I can simply pull the power plug, though, and this seems to force the machine into a different sleep state that can be woken for a single key press of the Space Bar (or any key probably).
It's simple, but it will save hours of frustration (OK, minutes and a lot of work) if your machine doesn't wake up in the middle of twenty projects (as mine often is). Normally I would have had to push on the power button until the machine died, losing all my work, in order to restart it.
[robg adds: There are a few other hints here about recovering from a non-waking condition.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060719112243990