This is awesome. Whether you like macs or not, MS should not monopolize the PC gaming market.
As far as marketshare goes, I believe the linux share is much higher than anyone realizes. Most linux users run a dual boot and desperately want to use linux exclusively, but can't get over the need of a few win apps.
If games start showing up on the Mac then they can very easily be ported to linux as I'm pretty sure both would be using OpenGL. I just hope more interesting game developers take the same route cause I don't care for most EA crap.