I think Live will hold up quite fine. As for new sign-ups, I think it will be the mega-killer app that Live's been waiting for and will probably be directly responsible for at least a quarter of what the game sells in North America this year alone in new subs. Regardless of how it goes, it'll be exciting and MS will certainly try to capitalize on its hotness.
Personally, if MS really wants to push the numbers on Live higher and expose the service to many potential players, they'll include a 1-2 month free Live sub with the game...announce a Halo 2 XBOX console bundle w/ Live starter kit...and release a Halo 2/Live starter kit bundle.