I respect Macs, but higer-end gaming is one area where Linux will succeed. Why? The video cards. The -best- video card you can get for a mainstream Mac is an X1900XT. The next step down is a 7600GT and finally a 7300GT. The only card that blows all of these away is a $1500 Quadro.
Linux on the other hand, supports -all- video cards. I am running Gentoo on an 8800GTX at the moment. We should be seeing support in Apples for that card in around 5 years ;-)
If emulation becomes a huge thing on Macs, then I could see companies shifting their focus there, and pushing out more video cards. Because at this point in time, -no- video card for the Mac would play a brand new game at full resolution, irregardless of how good the emulation is.
MacMan, you mentioned Trans Gaming, and I agree, they are pushing out some impressive stuff. Don't forget Wine though either, a completely free Windows emulator (of which Trans Gaming utilizes in their engine). Last night while bored, I actually managed to get my favorite MMO (Asheron's Call) of five years working:
http://deathspawner.net/aclinux.png
This was the fourth time in over three years that I've tried, but this is the first time I've actually been able to make it inside the game and play it at full speed. It just goes to show how fast we really are moving along. The game still has a few issues, namely with core Windows features that Wine doesn't support yet, but progression is evident.
As for Trans Gaming, I have been using the upcoming Cedega 6.0 for the past few days, and it runs both NFS: Carbon and Oblivion nicely. NFS: Carbon runs just as well as it does on Windows, but Oblivion is a little more finicky. Once you make it into the game, it runs really well though.
I think game developers should begin to get a clue and start coding their games for more than just DirectX though. If Linux has native clients of big games like Unreal Tournament, Doom III and Quake IV... why don't other companies follow suit?
I am not trying to compare the Windows market to the alternative OS market, because they dwarf it. It would just be nice to see some better support, so people wouldn't always have to run a VM or reboot their machine each time they want to run something.