tanmeatloaf, or Sebastian, over at Xbox365 just got back from the Xbox Unleashed event in Los Angeles and has posted an article about his experiences there. Halo doesn't get mentioned much in the article itself except to say that despite earlier indications, Halo was available in the free play areas but not used in the competetion phases of the event. Later, in a response to a reader question about Halo, he wrote:

Halo rules. i played very little of it because i had the competition going on. then again, I did play a bucket load of it at e3. Don't worry though, all the problems with the framerates have been fixed, and it looks better than ever. I can't wait to get it in my house.

Considerably more digital ink is spent on his meeting Seamus Blackley and getting an opportunity to dis a prominent GameCube supporter, but you can check all that out yourself if you like.

UPDATE: This story from two days ago at xbox.ign.com that explained the structure of the Xbox Unleashed event also mentioned that Halo was not included in the competition because of the 'M' rating. There's no information on exactly why it was allowed in the free play area, then, or how access to it was policed, if at all.