I have never beleived there was any real problems going on between Edward Norton and Marvel, I really didnt. I knew that Norton was a little afraid of crossing over the Marvel films, although he did find the idea fun.
Well a really good source (who wants to be left unnamed), tells me the real back story between Norton and Marvel's problems is this.
"Norton was originally going to cameo in Iron Man as Bruce Banner. Norton was really interested and was looking forward to doing so because he felt it was a great idea to have characters from Marvel to interact with one another like the comic books, and he looked at it as a great way to promote The Incredible Hulk.
But what ended up happening was, Norton's cameo was dropped for Samuel L. Jackson appearing as Nick Fury, which really pissed him off. He thought it was a stupid move not to have Banner appear since it would have built up hype for The Incredible Hulk."
If this is true, then I can actually understand why Norton would be hesitant to join the cast of The Avengers. It would also explain why Norton did not want to promote The Incredible Hulk for Marvel!