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If you find your Mac won't go to sleep, check out the results of this command:

pmset -g assertions

I found that Internet Sharing was preventing sleep:

$ pmset -g assertions
03/08/2013 21:19:08 BST
Assertion status system-wide:
PreventUserIdleDisplaySleep 0
PreventSystemSleep 1
PreventUserIdleSystemSleep 0
ExternalMedia 0
UserIsActive 0
ApplePushServiceTask 0
BackgroundTask 0

Listed by owning process:
pid 47(InternetSharing): [0x000000080000012c] 04:18:36 DenySystemSleep named: "com.apple.InternetSharing"

So I turned off Internet Sharing...

OSX:WontGoToSleep is mentioned on: OSX