Marty The Elder dropped into the HBO forum to answer questions about the Soundtrack promotion at Sumthing.com. Basically, the soundtrack to be released on the same day as the game will have whatever tracks of the game's score that are ready to be released at that time, plus "music inspired by Halo 2" produced by other artists. Although the list of bands has been removed from the Sumthing.com site at this time, Marty goes further and mentions that it looks like Steve Vai will be on the album, but Justin Timberlake will not.

In addition, some of this material, originally envisioned just to fill out the first soundtrack disc, is actually getting used in the game. Here's what Marty says:

Once I did the session with Steve [Vai], which I’ll tell you about sometime, I decided to trust Nile to fill out the Halo2 Soundtrack Volume One any way he saw fit. I know that I’ll like at least most of what’s on there. Two of the bands that worked with Nile and contributed to the album also sent me instrumental mixes of their stuff. The music was based on my themes and I was pretty blown away by them. The deal was that I could use those pieces in the game at my discretion, and that’s what I did. I like how I used them, maybe you will too, we’ll see. Trust me, they’re an amazingly small percentage of the overall musical landscape of Halo2, but I think they work great.

So there you have it.