I have been searching for a few days now an cannot find a thing.
I have an ipod touch 4g and have just started adding Game Center to a game i\'m working on.
Had the same problem. This worked for me: 1. log out GC, UNINSTALL APPSYNC if your're jailbroken, kill all processes. 2. create adhoc build, sign it with an appropriate entitlement, upload to your webserver, whatever. 3. download/install this ipa to your device and run, when asked create new account, or use existing - it will be registered (or created if new) inside sandbox.