First noted by Massa of Halo Network was an item at the German gaming site Babelfish version seems good enough to understand, so take a look at this:

Microsofts PR man Boris cutter Johne was yesterday to guest with Giga Games. A topic was the erscheinungstermin of halo. Afterwards the Xbox version of the play is published at the same time with the console in the fourth quarter next yearly. The PC version will follow unfortunately only three to four months later. During the conversion the emphasis is to be on Multiplayer. It remains being waiting whether up to then the play is still so revolutionary, as all hope.

If this information is to be believed, then Halo will be coming about three our four months after the X-Box's release. Some had hoped that Halo would accompany the X-Box's release like Sonic the Hedgehog (Sega), Mario (N64) or Combat (The Atari 2600, if you want to get really old) did for other systems.