I've been having this problem with my Powerbook for some time now. I have set the Energy Saver to various different settings for the hard disk to go to sleep, but it will never go to sleep on it's own. The only way it will go to sleep is if I manually make it from either the menu or by closing it. It's strange because if I am at the login screen, and don't login for awhile, it will go to sleep. I have also tried creating new accounts, and have had the same problem.
If anyone knows how to fix this or has any ideas on how to fix it, I would appreciate it. Thanks.