usually people say that cross-platform play brings more trouble than it's worth. it's a pain to code and leads to unbalanced games. they did it in quake 3. console players pretty much always lost. I know that's a blazing fast FPS, but those can be played with gamepads even if they can't be won against anyone who doesn't suck. RTS's have a long way to go on consoles. few people actually are proficient with any reasonable control scheme. the xbox crowd spend all its time playing shooting games and single player games that aren't RTS's, whereas the people buying the PC game will know how to play strategy games - both in tactics and in controls. halo wars might help a bit, but i don't think it will do all that much. the other RTS's have either been voice activated or PC ports.
however, I don't see this game being very popular the moment it comes out. the online community will probably start small on both versions. i hope it's active enough to sustain itself so that people can find whatever match they want whenever they want, but realistically you can't guarantee that for this game, so you might need cross-platform play just so there are enough players for the console crowd to play against. that might be the only way for the game to succeed on the xbox unless people buy it for the single-player campaign alone, but MMORTS doesn't imply that.
other than that prediction, i just don't like the idea of the same strategy games on consoles and PCs. it usually forces games to be dumbed down as it becomes too simple and easy with a low unit cap (consoles have worse processors and less RAM than any cheap modern PC) to be a game that won't sell unless it appeals to the console audience which doesn't know the first thing about an actually strategic game. console strategy games have their own place (the watered down civilization revolution, the voice activated end war) and those can come to PC if necessary, but in general the PC version of a game gets hurt if it goes to consoles. perhaps there was some point during the development when developers rejected a feature because the xbox or its players could not handle it. if you can really make the same game be as good as possible on its own merits on PC and playable to some degree of competence on consoles it will be a significant achievement.