It just means there won't be a Hulk 2 and if Norton doesn't want to do Avengers, Marvel will just write the character out. Which is too bad, because I really enjoyed TIH quite a bit (even if it ordered on dumb fun, it wasn't insulting) and really was looking forward to seeing Tim Blake Nelson play an evil genius in the sequel.
Oh well.
Even if Norton would not do the Avengers why would Marvel write the Hulk out? I would think he would be the easiest to deal with in that kind of problem accorded, because isnt the Hulk CGI. Sure I want Norton to play Banner but if that didnt happen all they would have to do is reduce the part of Banner to very minor one and have most of the scenes take place after he hulked out.