If you're looking for some solid information on Halo 2's top-secret plot, then head right on over to TeamXbox, where they've interviewed Frankie about it and been told... that it isn't being discussed right now.

Interestingly enough, that was in response to a question about online coop. So at the very least it appears as if the answer isn't a definite no at this point; more information is promised. Soon.