That's quite a casting merry-go-round for one character, the kind we haven't seen since the revolving door of Batmen in the 1990s. The plan is to include the Hulk in the upcoming Avengers movie, and possibly follow that film with another Hulk sequel (will we still get Tim Blake Nelson as The Leader?). There've been some rumors that the Hulk would be the thing that unites the Avengers (Captain America, Iron Man, Thor, and others), keeping the story from the very first issue of the Avengers comic somewhat intact.
I'm glad to see Marvel is so committed to the cinematic success of one of their most easily recognizable (and thus, most merchandisable) characters, because I'm just a huge Hulk fan. I've collected comics for nearly thirty years, and out of my collection, I own more issues of The Incredible Hulk than any other title. I grew up watching the television show, and I think most little kids glom on to the Hulk as a character because he's a big, green destructive force of nature -- one that's dumb enough for them to feel smarter than, and primal enough for them to emotionally connect with.
There's some who feel that it really doesn't really matter who plays the Hulk, because he's the star and not the mild-mannered Bruce Banner alter-ego. I can understand how that appears to be true, but, honestly, the Hulk has yet to appear in a film where Banner's personality is accurate to the comics.
One of the things Ang Lee's film does right is make Hulk and Banner feel like the same person. In Hulk's more emotional scenes, you can see Eric Bana's face in the monster, and it's that movie's single biggest strength as a Hulk film. It's just too bad Eric Bana's Bruce Banner is so vanilla. They explore his relationship with his father, something brought to life in the comics primarily by writer Peter David, but where David has Banner internalize his emotions as a way of dealing with his murderous dad, Lee's film makes Banner a middle-class orphan, raised in a well-adjusted home where he never knows a thing about his dad (or that he's even adopted). This white-bread Banner doesn't grow up with any emotional issues, so the film has to shoehorn in another way to unleash the Hulk character -- by having Banner get injected with a mutagenic DNA stew as a baby. When the gamma explosion happens in the film, it's only the catalyst for Banner's already-dormant physical mutation, not the event that unleashes the beast from within his own psyche.
Instead, Banner is portrayed as a nice guy with a hair-trigger temper. That temper happens to cause him to change into the Hulk, so he tries his best to hold back anytime he's angry. This is a huge difference from the way Banner is characterized in the comics. It's simple to say that Banner's metamorphosis is caused by anger, but the reality is that Banner panics when placed into situations that negatively affect him. Banner's anger comes from his own lack of control over his panic, not from the outside source itself. It's a tough thing to show on-screen, but it can be done. It's the difference between "you made me mad; now I'm the Hulk" and "you made me feel helpless; now I'm the Hulk." The Hulk is unleashed to affect the change that Banner is incapable of -- this is also what makes the Hulk a hero, and not a Godzilla-type unstoppable monster.
Norton handles the character better than Bana does, but still isn't given the inner conflict that's so crucial to the part. Norton's Banner is a calm zen master, avoiding as much contact with others as possible, while trying to find a way to rid himself of his curse. The movie finds some truth in Banner's acceptance of his alter-ego at the film's conclusion -- that it may be important for him, specifically, to be the Hulk over anyone else. Again, this is about Banner's ability (or lack thereof) to affect change. When he can't do something himself, the Hulk does it. If someone else was the Hulk, they may not have the moral compass to right the wrongs that Banner sees (this is made explicitly evident in the story of Emil Blonsky, played by Tim Roth, who also becomes a super-strong monster in the film).
I'm more concerned with how the character will be written on his third cinematic at-bat than I am about who plays him. There's some rich emotional stuff to explore in Banner's story, and Lee's film, frankly, screwed it all up, while Leterrier's placed a heavier emphasis on crowd-pleasing action (mostly to spark the franchise that Lee's film was too dry to do). Ruffalo may be jumping into the purple pants as a capable replacement, but, if Zak Penn is writing Avengers, then I'm resigned to the fact that I'm still not going to see Banner portrayed the way the character exists on the comic book page. If they can distract me with just the right amount of smashing, I might be okay with that, but I'll still be holding out hope for a more accurate movie Hulk.