And there's also a new Hulk movie in development with Mark Ruffalo signed for the role. With that kind of schedule, you've got to think that Avengers 2 could be about 3 years out at the very least.
This isn't 100% accurate... Ruffalo was signed to reprise the role of Banner in 6 films, but that does not mean a Hulk film. He could show up in IM3 and he'll surely be in Avengers 2. I see many making the leap that there will be a new Hulk movie, but past box office performance makes that a very risky proposition.
Many times, a studio will lock in an actor for several films "just in case" so that if they need him, he is obligated to perform. They could just as easily tear up the contract. I believe Norton was signed for multiple films as well, but we all know how that turned out.