I doubt that he uses some unreleased Mac. Think about it, the man has to work on it, it has to be stable. I guess a regular MacPro. Maybe somebody came by and said "Hey, we got those Becktown 8-Core samples", that's the best I could imagine - and that would be the upcoming MacPro (if there wouldn't be SandyBridge). Remember, they just bump CPUs, completely new designs only come every couple of years.
Sure he got the good NDA-stuff, but just to play around, as it might not be stable.
It's just like with the rest of us. If you need stability, you wait for .1-versions.
I think he uses the iPad II already, though - if it's already usable.