Ok, this time it is for real, because it's from the horse's mouth, which is certainly the most trustworthy orifice on a horse. Bungie.net says that Marathon: Durandal is being brought to XBLA by none other than Mac game developer Freeverse. They have a page on Marathon: Durandal at their site.

It will include single player, eight player cooperative play, and multiplayer.

Eight player coop? Zang!