`pmset -g assertions`: find the processes that are killing your system’s ability to sleep.

$ pmset -g assertions

12/22/12 7:36:04 AM EST  
Assertion status system-wide:
   PreventUserIdleDisplaySleep    0
   PreventSystemSleep             0
   PreventUserIdleSystemSleep     1
   ExternalMedia                  0
   UserIsActive                   0
   ApplePushServiceTask           0
   BackgroundTask                 0

Listed by owning process:
  pid 46129(AddressBookSour): [0x0000000100000fd0] 00:00:03 PreventUserIdleSystemSleep named: "Address Book Source Sync" 

$

Take some caution, but it might be a good idea to kill any identifiably-zombified or long-running PreventUserIdleSystemSleep-related processes. They, uh, prevent sleep (and drain battery life).