My PowerBook under 10.2.8 all of a sudden refused to go to sleep. After some hunting, I figure out that the problem was that I had configured the Desktop control panel to randomly change the background (using the abstract collection) every minute. However, I had the Energy Saver configured to go to sleep after four minutes. Setting the Desktop control panel to change the background every 15 minutes curred my sleepless Mac. Looks to me as a bug.
[robg adds: I'm not sure this is a bug, but it's certainly something to be aware of -- something as simple as changing the desktop background will certainly make the Mac think it's still doing stuff, so Energy Saver will not kick in. Just think of it as one more thing to debug if you're having sleep issues.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031010041144491