Do not take this as definite, and this very well may be too good to be true. A close source that is part of helping with making Comic-Con 2010 has told me a rumor he has heard around the convention.
The source said: "There have been numerous talkings around here that Ed Norton may be appearing for The Avengers panel. Contrary to Kevin Feige's "confirmation" of Norton being out of the film, it looks like it was all to create buzz and controversy. I have heard some reasons why this is very likely true, 1) Marvel Studios did not put out a press release about Norton leaving The Avengers, and that no movie studio would openly e-mail some website confirming anything for a movie. Especially when that website reporting the news says it without Marvel Studios permission. 2) Marvel.com would have at least had something up about it, but no, nothing. 3) Even though it was reported that Mark Ruffalo would take over the role, and even his agent came out saying he was in some talks, likely we would have heard something since if talks were going anywhere."
The source went on to say that Marvel Studios would NOT recast one of the main actors. Marvel wanted to make use of all the rumors surrounding Norton/Marvel having issues and use it to stir up some buzz by "firing" Norton. It is an attempt to create some more interest in a possible "Incredible Hulk 2."
Could this be true? Hard to say, but we should know on the 24th (?) if Norton is in fact in the film and his being let go was all a hoax. Assuming this is true, Norton should be joining actors Robert Downey Jr., Chris Hemsworth, Scarlett Johansson, Samuel L. Jackson, and Chris Evans on stage to discuss The Avengers.
Also, I am very sorry for my lack of being able to post sources.