Hi erebusman,
i'm working on Mountain Lion since we have the preview and all is ok.
I added the MAC Game Center to all my games and it run very well.
Also some of my games have the multiplayer coded with Game Center.
Yes you need some adjustment to get apps work in it.
Starting from the ICNS file iconset. But for this i just uploaded my new app to build it auto.
You can find the post on the main category of AGK.
Anyway, i will release some PROMO codes for TGC/AGK friends.
If you need help on something don't esitate to ask.
I'm using 1076 AppGameKit and all is fine.
Long life to Steve!