I think the issue is with the LaunchAgent "Antipop."
I had this no functional auto-sleep issue on 2 of my macs, the two with Antipop installed. In my long search to fix the sleep issue in the MacBook & Mac Mini I thought I had successfully UNinstalled Antipop by manually removing what I thought were all the parts to that LannchAgent. However, I still had no autosleep.
(console gave no hint that Antipop was keeping the Macs awake)
Just last week, on a tip, I decided to use the Un-installer script that comes with the latest version of Antipop.
But, at the same time I did happen to install the newly-released SL Security Update 2010-001.
Restarted, and for the first time since 10.5.something, functional auto-sleep was back! I blogged about my ordeal, should you be interested in the details, and continue to have issues with auto-sleep on your Mac.
Sorry to bump an old thread, but this was a near-year issue with my Macs, and there is little information to be found on this specific fix...The Original Poster even stated they had Antipop installed on their rig...