Hi,
I dual boot a couple of my Macs (quad boot one of them, OS X, Vista, Win 7 and Ubuntu ). I will try a render on Mac and Windows but even on the same machine I would expect there to still be differences. I don't think Windows and OS X share the exact same "numeric environment", if you see what I mean. Even above the processor level the OS still has influence over the way numeric computation is done.
Regards,
Jo