Bungie used to develop games exclusively for Mac. Apple asked them to build a killer game that will give 3D gaming on the Mac a nice start. The original Mac-only build of Halo was shouwn at the Apple Macworld Expo in 1999. Then, Microsoft liked the game so much that they purchased it. Too bad. Gaming would've really took off on the Mac if it was an Apple-exclusive. Just letting you guys know this before you finish the fight on your Xbox 360 instead of your iBox 360.
Here's the original keynote (in horrible quliaty):
Footage of Halo on the Mac: