I have had an intermittent problem with my Titanium powerbook not waking properly from sleep: the hard drive wakes up, but not the screen. I don't know what causes it, but I seem to have discovered a workaround: if this happens, close the cover, wait till the pulsing light at the hinge indicates the machine has gone back to sleep, open it up again, and hit the spacebar. This has worked for me twice so far (it doesn't happen very often, so I haven't had the opportunity to test it more than that).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020815092807966