Per the valve guys in an interview with AppleInsider, Apple provided quite a bit of support and help when they were porting Steam/Source to Mac.

It makes sense - Apple has been interested in Macs as a gaming platform for years, but has never been serious enough to put their considerable muscle behind it.

If someone else is willing to throw their money, time, and influence at the problem, all the better for Apple.

The interview: http://www.appleinsider.com/articles/10/03/08/valve_apple_wo...

World of Warcraft has been running on OSX for several years, and Apple has helped them to get better performance over time.

Indeed, a couple of OS X updates have mentioned OpenGL changes made specifically to boost performance in WoW. For instance, http://support.apple.com/kb/HT1525 "... improves the reliability of OpenGL-accelerated graphics in Blizzard's World of Warcraft".