I can see this being possible, if they are speaking of the XBox being the exclusive platform for a given period of time. Anyone who follows the console news heavily will know that there has been talk of MS courting Valve into an exclusive deal for Half-Life2 for some time. At one point(perhaps still) HL2 was listed as a XBox preorder item over at EB(nothing for the PC, far from concrete obviously but combined with everything else). I think that based on all we have heard MS has at the very least tried very hard to have HL2 XBox exclusive for a given period of time. It could also make sense if they are using an engine that would require DX8 level hardware to run. In the PC market, requiring DX8 would kill off a
lot more of the potential market then requiring a nV board would(noone likes to talk about that though ). If Valve wanted to use a high end engine to power their game signing up with MS for a year long exclusive period, taking the cash from MS in the process, and allowing them potential multi-million sales levels before they release it on the PC when DX8 hardware is more commonplace then it is now doesn't sound like an impossible deal.
Furthermore, the chances that HL 2 will be Xbox only is about ZERO.)
Bungie was a Mac focused developer whose killer app was to elevate the status of Mac gaming and make the platform more attractive. Halo has since sold over three million copies on the XBox(~$150Million dollars in revenue generated) and between the Mac and PC combined managed zero(if Halo doesn't do well on the PC, expect more PC centric developers to be willing to take MS/Sony/Nintendo cash for exclusitivity deals). While I do think the idea that Half-Life will remain XBox exclusive is slim, the odds of Bungie becoming part of MS was quite a bit smaller still.