MoBill on the H.borg forums pointed out a link to an article at PCFormat magazine. According to the article, Microsoft has confirmed that Halo will be released for the PC as well as the X-Box. From the article:

Microsoft actually rang us yesterday to give us the bottom line on what's going on. Bungie will *definitely* remain an independent group within Microsoft - getting access to the big M's marketing clout and cash pile - whilst Gates' company gets some top game talent. Halo will still arrive as planned on the PC - but will also be released on X-Box.

There is as yet no confirmation of this from Bungie, as well as no mention at all of the Macintosh version, which only heightens the irony that Halo was unveiled almost a year ago at MacWorld in New York, running in real time on Macintosh hardware.