Shacknews have posted up an extract from an interview with Ensemble's Dave Pottinger in which he talks about the cancelled Halo MMO. It turns out the model was very much "World Of Halocraft", as people had suggested.
Shack: So you didn't have plans to go for a more FPS-style game.
Dave Pottinger: It was definitely more of a classic MMO, but it still had that very action feel. Definitely more actiony than WoW.