Mac, But I probably will be using my Win7 partition
I don't mind really, if they were to port it to mac it would probably be slower and released a lot later...
I do support the effort of Valve: releasing their future games at the same time as the pc version and also with good performance. Only if those 2 things are in order, an OS X version is useful.
MacbookPro 5,1:
2,4 GHz
3 GB
9600M GT
I hope the new engine is optimized and not only high-end minded like Crysis.