According to the March issue of OXM, Steve Vai's guitar-charged "Halo Theme (Mjolnir Mix)" could be a downloadable track for Guitar Hero II. Heck, if Spartan-458 can jiggle in Dead or Alive 4, then Halo 2's theme can be strummed along to in the GHII encore. This is one Halo rumor that smells probable; if not simply because it screams, "impulse buy!"
Assuming we'd have to pay to play, the Halo theme has the potential to pocket as many micropayments as just about any licensed track imaginable. This is the Xbox 360 venue, folks, where Master Chief is as big as any 'Rock God' -- dead or alive.