I would perfer it on PC (or with keyboard and mouse on 360) but I played Battle for Middle Earth II on the 360 and the controls aren't nearly as bad as you would think and the other RTSs console games. The only problem it has is managing multiple large battles. I hope they don't screw up the controls. But it's going to stay 360 exclusive for now. I wish they would release it on PC, but knowing microsoft if they do that it would require Vista like Halo 2.
I never hear people complaining about pushing the franchise too far when franchises like Warcraft and C&C changes genres and story lines and just adding in new gameplay and graphics. The three Halo games have a direct connection to each other and a continuation of the story line. A third person MMORPG is pretty different from a RTS, and the same with making a RTS franchise a first-person shooter (Renegade). I've always thought it would be cool for a Halo RTS but I wasn't thinking it would be console exclusive (I shouldn't be surprised). But then again this is when I never played BfME II. I must say even if it is pre-rendered the trailer looks real nice. I hope Halo 3 looks like that.
Intrestingly enough if you know the development history of Halo you would know it was originlly designed as an RTS on the mac. Well Ensemble Studios is making it, and they made the excellent AoE games so I think they won't totally screw it up.
EDIT: I read the faq and the trailer is pre-rendered. Oh well I'm sure it would still look really good.