Bungie have not even announced that they are working on Halo yet.
Halo is the XBOX killer app the product that sells the most (well a lot of) consoles. M$ will want this to be a show case app. To show what next gen is all about. They are in no hurry to release the game because there is still a huge market demand for the console itself and they do not need to make supply issues worse by releasing a must have game when there are not enough consoles to go round. Yes supply is getting better but currently it would become impossible to get a 360 if halo came out next month. By the time there are enough consoles to go round £5 will be knocked off Halo's price tag.
As I was saying about Halo being a next gen show case. I would imagine they are probably still working on the physics engine, the graphics engine and the AI. They will be working on the multi-threading and truly getting to grips with the hardware. Halo has to look better than anything currently available, it has to play better than anything available, it has to sound better than anything available, and it has to feel better than anything else available.
As I have not seen a single screenshot yet I am sceptical that this is anywhere near completion. I think we were seeing Halo2 screen shots in February when the game was released in November.
There has been no details on weapons vehicles or any other details about the game at all, infact the only details we here are suspect release dates. Why would M$/Bungee not let us know anything if they were near completion?
Things I would imagine are complete:
Storyline
Voice Acting
Sound Track
Sound Effects