Jason Loonyboi Bergman from Shacknews recently interviewed Chris DiCesar, Microsoft Games' group PR manager, about the status of the PC/Mac ports of Halo. In terms of when it will happen, DiCesar said it's still up in the air . He also indicates that no actual work has even begun on the port:

Jason: So itТs not on the board at all at this point?
Chris: No. Well I would say itТs on the board as planned to do, but in terms of actual development of it, it is not.

There's little concrete to point to from Matt or anyone at Bungie that directly contradicts this, although it certainly is disheartening to hear that at this point no actual work has been done, more than six months after the release of Halo on the Xbox.