Rock, Paper, Shotgun thinks that a Halo MMO might be coming. Writer Jim Rossignol comes up with a laundry list of reasons why, some of which I don't find entirely convincing.

I'd probably play it if they made it, though. I originally thought Halo-- the first game-- would be something like PlanetSide. It'd be nice to finally see a game like that in a universe as compelling as Halo's.