Assault Cube on a Mac

[MathiasB]

I forgot when this started, but macs have two GPUs, one integrated and one not integrated. It switches between these as needed. I would not be surprised that the fans turn on to cool down the non-integrated GPU, or if it didn't switch it would definitely need to cool down the integrated. All is normal, if anything you should start getting worried if the fans don't blow.

That's correct for al 15 and 17 inch MacBook Pro's.
I've a 13" MacBook which has only an integrated GPU. And a Mac Pro which has of course a discrete GPU