Same problem with my iMac. The problems seems to come from the audio subsystem in my case, sometimes usbmuxd, sometimes coreaudiod :
# pmset -g assertions
9/1/11 6:12:08 PM GMT+0
Assertion status system-wide:
ChargeInhibit 0
PreventUserIdleDisplaySleep 0
PreventUserIdleSystemSleep 1
NoRealPowerSources_debug 0
CPUBoundAssertion 0
EnableIdleSleep 1
PreventSystemSleep 0
DisableInflow 0
DisableLowPowerBatteryWarnings 0
ExternalMedia 0
Listed by owning process:
pid 169: [0x0000012c000000a9] PreventUserIdleSystemSleep named: "com.apple.audio.'AppleHDAEngineOutput:1B,0,1,2:0'.noidlesleep"
Kernel Assertions: 0x0004
* Kernel Assertion ID = 500
Created At = 1/1/70 1:02:27 AM GMT+0
Modified At = 8/31/11 5:50:46 AM GMT+
Owner ID = 0xffffff8013d81000
Level = 255
Assertions Set = None (4)
* Kernel Assertion ID = 501
Created At = 1/1/70 1:02:33 AM GMT+0
Modified At = 8/31/11 5:50:54 AM GMT+
Owner ID = 0xffffff8013dc2000
Level = 255
Assertions Set = None (4)
# ps aux | grep 169
_coreaudiod 169 1,9 0,1 2516412 7936 ?? Ss Mer03 3:50.76 /usr/sbin/coreaudiod