I'm tempted to say never. Linux (and other unices) will never be compatible with a crucial element of modern commercial games:

Copy Protection

With the combination of 'everything is a file', well locked-down permissions, and the complete forbidding of direct hardware access, there is no way for copy protection to work. The 'copy protected' app has no way to tell whether it's reading from an original CD, or being misled by the kernel. With all of the core components of the OS being open-source, it becomes easy to tell the game whatever it expects to hear -

'am I running as root?' Sure!
'Is that an original CD in the drive' Of course it is!
'I see you have no internet connection?' Don't even have a network card!
'Hope you're not running a debugger?' Nope!

and happily enough, the game runs. From the game's point of view, the system is a black box, with no way of getting at the underlying hardware (or even knowing for sure what it is) without r00ting the box. The only protection system that has a hope of working under linux is a Quake III-style online CD key system, but how much of a pain in the arse is that when you're playing a single-player game? (not to mention, how easy is it to crack, when you can completely hide a debugger from the program being debugged?)

The only hope for gaming on linux is either Free games (apt-get install nethack), or widespread acceptance of running games as root (and remember the fuss with games only running as administrator under windows 2000?).