PunkRocker reported in HBO's forum that Daily Radar has slapped up a Gamestock preview, and Halo is the first on the list.

Bungie
Halo
Although the game is still being constantly refined, it's no secret that Microsoft will be showing a playable version of the game running on the Xbox. There are still plenty of mysterious surrounding the game -- such as how much is played in first person and how much in third person -- so Halo remains the ace in Microsoft's hand.

Gamestock is due to occur on the 13th of this month... I'd pop you a URL, but Microsoft doesn't appear to have a public site up yet.