Do you need to release GCD queues under ARC in iOS 6.0?
When you target Mountain Lion and iOS 6.0, ARC will now manage dispatch queues and other GCD types for you. If you target earlier versions of OS X or iOS, you will still need to explicitly retain and release GCD types (not just for thread safety reasons, you'll leak memory otherwise), but this is handled for you when only targeting the newer OS versions.
This is why you see such a compiler error under ARC when targeting iOS 6.0.