I'm fairly certain Grand Theft Auto: San Andreas will appear on the PC (and probably X-Box) somewhere down the line. None of the ads imply that it is a PS2-exclusive title.
As for Halo 2, I would imagine it will also make it's way to the PC, but I'm not positive. I don't think Halo did nearly as well on PC as it should have (something I'm sure Microsoft are happy about because it means their practice of only releasing a title for their own proprietary hardware system in order to boost sales of said system so gamers [i.e. consumers] will be likely to purchase even more games for the twice mentioned system in order to "get their money's worth" [read: justify the initial hardware purchase by using it often with many different softwares]).
I see no reason why they would want to put Fable on PC within the next two-three years (at best). It's in a great position to provide the X-Box with a second wind, one which it desperately needs while Sony and Nintendo are in the news on an almost daily basis with their upcoming systems.