I have the same problem. It always takes about 30 seconds for my Mac Pro to fall asleep and it's driving me crazy.
I've got a Mac Pro early 2009 4.1. I've reinstalled the OS twice, disabled all startup items, uninstalled the Logitech Control Center, and unplugged all USBs. Only running keyboard and apple Mighty Mouse on bluetooth.
I've also disabled hibernate mode, and removed the sleep image from the boot disk.
On a pmset log, every thirty seconds the "com.apple.diskmanagementd" program creates a "PreventIdleUserSleep", then thirty seconds later it creates a "ReleasedIdleUserSleep". This repeats itself continuously while it's sleeping.
I've tried killing the process in Activity Monitor and it just starts up again 30 seconds or so later.
What could be causing this, and how can I stop it?