I don't think there's any reason the rooftops would be off limits. There are no invisible walls in Brink, so there's no build in height limit. And developers made sure that if an object in the environment looks like you can interact with it, then you can. So the only way a rooftop would be off limits is if they made it simply too high to get up there. But seems like such a ploy would just lead to people figuring out exploits, with the lack of invisible walls.

Besides a concrete release date, I would like to see some more gameplay footage from some different levels we haven't seen yet.