I'm sorry that Norton won't be back, because he's a great actor and also for the sake of continuity.
But you know, part of me admires Marvel for eschewing the usual Hollywood 'we had a great time with Ed but the time was right for a new Bruce Banner yadayadayada.' Standard bullshit. Instead, their attitude seems to be 'we're the new production company in town, we're not doing things the Hollywood way, if an actor dicks us around, he can f*ck off.
No-one is indispensable, seems to be their motto. Now, admittedly, I think they'd have a problem replacing Robert Downey Jr, but then again, The Incredible Hulk (which I enjoyed) didn't exactly do Iron Man-type business, so I suppose they'd rather take the flak from fans for now and trust us to turn up to see the movie anyway. Which seems to be what most of us are going to do.
Norton is a great actor but he has form for being difficult - American History X and The Italian Job come to mind. I'm sure Marvel are no saints either, but I can imagine that with an ensemble cast, a movie that is crucial to the future of the Marvel movie-verse and a talented director who is nonetheless a newbie to a project of this scale, they just didn't want to take risks on him kicking off.
I want Kiefer Sutherland as Banner. Imagine him saying 'Don't make me angry. You won't like me when I'm angry.' And he's done the fugitive-thing very well on 24. While most recently known for Jack Bauer, he does project intelligence and I think could convincingly portray a scientist.