Do a google search on this topic and you'll find you're not the only one with this problem
I had it too. I was never able to pin down the cause. My mac pro used to sleep as directed by the energy saver prefs. But I've changed some things on my system, none of which should ostensibly cause the sleep problem and none of which I'm willing to remove (my leading contender is attempting to use the DVI port of my 5870 -- yeah, I know, doesn't make much sense to me either).
At any rate I say "had" the problem because I ended up using a freeware utiiity called
PleaseSleep. I realize it's a workaround but at least the machine now sleeps at the designated time.
Note, before there is followups on this subject, I am no novice when it comes to os x. So
yes, I did all the obvious stuff -- proper energy saver settings, checking processes that are running, trying a separate pristine test user account, etc. Score 1 for the machine and 0 for me (or a "draw" if you count using PleaseSleep).
Bucko, I'll guess I'll ask the obvious questions though, or more generally you might want to read
this apple tech note.
