More from the interview with Loren Brichter:

> LB: It’s all custom. The whole game is actually written in OpenGL with my own animation system and a bunch of other neat things.

http://www.macstories.net/featured/a-conversation-with-loren...

I recall lamenting (well, only sort-of; last thing I need is a new addiction) that the use of GameCenter meant it's unlikely to be easily ported to Android (or at least an Android port wouldn't interoperate with iOS users). The funny thing is that, being written in OpenGL, it's be much easier to port to Android than if it used UIKit. Ah well.