Without citing a source, GameSpot Australia reports that Half-Life 2 has been confirmed for an Xbox release, "tentatively pegged for an October 2001 release." Oddly, the Xbox itself isn't due until November, and a PC version of the game also hasn't been officially announced yet, so you may want to take this with a grain of salt. Thanks Shacknews for the tip. Here is an excerpt from the story:

Half-Life, the highly acclaimed first-person shooter by Valve Software is set to re-invigorate players in the surprise appearance of Half-Life 2 on the Xbox release schedule.

This may indicate that other versions of Half-Life 2 may take longer to reach customers -- as is suspected of Halo, Bungie software's epic future warfare title. Halo was originally slated for initial release on PC and Macintosh, but since Bungie became a first-party developer for Microsoft, their plans have changed to favour the Xbox as a release platform for the game.

Update: Stomped reports that a unnamed Valve employee has refuted the story.