On the halo.bungie.org forum, Matt Soell was asked whether Halo will come out for the PC, and if so, why this announcement wasn't simultaneous with that of Halo's being released for the Mac. His response:

Releasing it for PCs as well is the obvious thing to do. It wasn't PCWorld Expo, ya know. :-)

Everyone comes to the Halo party.

Read the thread.