Sigh, I have been watching Halo news ever since the first Halo movie was shown on a 400Mhz Mac using ATi 128 video card! (don't ask me how). Since when have been number of movies shown of Halo running both on Mac and PC.
Show it's a big misunderstanding that Halo ever rooted on xbox, but Bungie was acquired by Microsoft and became Bungie Studios, a unit of Microsoft's gaming division. It as never has been totally clear what was in the deal expect it will come out for the PC, but at least 2 months after the xbox release.
For more info I recommend HBO (halo.bungie.org) it used to be most detailed site on Halo pre the xbox release.
But as for the release date of the PC version MS has the final word on that, so as long as Halo keeps seling xbox we should be expecting more delays on the PC release.