In the latest weekly update from Bungie, a fan asks: "Marathon and XBLA (referring to Xbox Live Arcade): is it at all possible?" Bungie, via Frankie, responds "Of course it's possible, it's just not on any current agenda. If it's any consolation, many, many people think it's a good idea."
It sucks that the statement is technically closer to a denial than a confirmation, but that won't matter: any time Bungie even mentions its legendary (originally Mac-only, I'll have you know) first person shooter series it's an excuse for Marathon fans to break out the bubbly. Just make sure you're not around when the fans swap the champagne for an M-75 combination assault rifle and grenade launcher after they realize that after they* stole Halo from the Mac platform for their* Xbox platform, they* could also be about to steal the definitive Mac game.
*"They" of course refers to a certain software company beginning with the word "Micro" and ending with the word "evil".