Marvel Avengers Movie with Iron Man and Hulk Coming Eventually...
This isn't really a rumor, but it's so far out and pretty much a big work-in-progress that it practically is a rumor. Comic book movie screenwriter Zak Penn, who's been very vocal recently (talking about Young X-Men spin-offs among other projects) shared some details on a few of his movies with MTV, including The Incredible Hulk and possibly an Avengers movie. The Avengers are a team of superheroes, just like DC Comics' Justice League of America, but comprised of Marvel superheroes, including Ant-Man, Wasp, Thor, Iron Man, Hulk, and Captain America, among others. Although Penn does say it's "definitely going to happen", he's waiting for Iron Man and the new Hulk movie, both summer 2008 releases.
Here's the quote from Penn himself talking about the film.
"'Avengers' is definitely going to happen; the idea was always to do a crossover with multiple superheroes, "he revealed of the superhero tag team group, Marvel's answer to DC's Justice League. "I'm waiting to let 'Iron Man' and 'Hulk' come out because those are two characters that could appear in it. I used to think that movie would be impossible to make but now I feel like it's not."
For a while, I too thought it would be impossible, but in the same fashion that they're currently developing a Justice League movie, I think if everything was done right, they could do an Avengers movie and make it one amazing crossover. For now we've got a while to wait, and let's focus Iron Man and The Incredible Hulk individually first.
In the meantime, you can also pick up Lionsgate's two animated movies on DVD: Ultimate Avengers and Ultimate Avengers 2, which are actually both very good and certainly worth watching if you're a big Avengers fan.