The Apple Developer Connection has posted an article
detailing various game engines and tools for Mac OS X game developers. In the spotlight is GarageGames' Torque Game Engine, OverTheEdge's Unity and Phelios' PTK engine. Perhaps there might be more to the possibilities of the Mac being a gaming system
down the road?
Check out Developing Games on Mac OS X Using Third-Party Game Engines to get started on that blockbuster game you've been kicking around.