From PlanetXbox AND PlanetHalo:

Q:What platforms will Halo be developed for, and in what order will they be released?

A:Halo will be developped for the Microsoft X-Box, the PC and Macinstosh . The X-Box version of Halo will be released first to promote the release of the X-Box, with subsequent versions of PC and Mac following shortly thereafter.

Halo on Xbox Release Date: November 15th, 2001.
Halo on PC & Mac Release Date: Early 2002

Q:Why does it Halo’s packaging say “Only on Xbox” on it? Are they not releasing PC/Mac versions?!

A:There will be Xbox, PC and Mac releases of the game. It has been stated and re-stated by Bungie many times, but some people still don’t believe it. “Only on Xbox” is much like movies advertise themselves as “only in theaters”, it’s just to promote the Xbox. I’ll quote Bungie’s Matt Soell to confirm this: I will take this opportunity to remind the more easily shaken members of the community that THERE WILL BE MAC AND PC VERSIONS OF HALO, no matter how many posts you might see from malicious dolts who “confirmed with a Bungie developer that Mac Halo is dead” or whatever. Don’t let anonymous jerks ruin your day with lies.

YAH!!! Now all I need is a computer upgrade 🙂