I think the whole thing is probably real. I can't help but wonder if the whole thing is a joke set up by Whedon, Feige, and Norton. So I thought I'd stir up some speculation.
Here are a few red flags.
1- Proximity to Comic Con
With the convention under a month away, one has to wonder about the timing. Keeping it close to the convention would minimize the impact, and make things easier to keep under wraps.
2- Method of Notification
So far we've heard from Marvel, but I've been told it's not posted on their official site, which would be normal. I'm not an expert here, editors or regulars feel free to confirm or deny the validity of this.
At the same time, we hear a response from Ed's agent. Not Ed himself. Norton's been invested enough in this process that I'd have expected to hear a statement from the man himself addressed to the fans. But nothing.
It feels like they're giving us the info through official sources, but not quite official enough that they'll be called outright liars or cause too much confusion or controversy.
3- Tone of Marvel's Statement
Marvel's statement seems a little too insulting to be official to me. It just seems like standard practice to give a vague "creative differences" kind of statement in these circumstances, so as not to burn bridges in the community. As stated in his agent's reply, Ed is good friends with a number of the people on the Avengers- and he has a lot of ties in Hollywood. It seems like a big company like Marvel would try to minimize the damage, and just say "look, sorry, but it's not happening" and not add any little comments or whatever.
So I dunno. Am I just wishing here? Time will tell. But I wanted to pose the question.