Game developers won't bother even considering porting games to OSX if Apple offers nothing that can handle their games to their full potential.
And I don't mean "well, for 3mths every 2 years they have a semi-top end card available for mac".
I agree, but you'd have to see a sea change on the drivers from for this to happen.
We all know you can run an 880 gt in a mac pro today...providing nvidia released an osx 10.4/10.5 driver for it.
I guess the key here is does apple think that people are going to upgrade their computers less, if it's possible to do small incremental upgrades of ram and gpu that today where it's not.
So does apple go into the selling gpu business far more than they do today where they only really offer post sales bto options after the event.