Exactly the reason why they must do solo movies before they make the Avengers. Getting origins out of the way save a lot of screen time for other plots like the villain's origin. Actually if the main antagonist is Hulk like in the The Ultimates they can do away with that too.
I got a feeling that DC is waiting to see how Avengers movie turn out before deciding their next course of action. If it's a huge success expect JLA movie to be released within 5 years. I just hope they know how to translate Thor to the big screen successfully. He's the only superhero with an origin that's too unbelieveable to be made into a movie, but if the audience can accept Superman's origin then I am sure they can for Thor as well.
Thor story is almost easy to tell, it's pretty much Norse Mythology. If people can believe in Jason & the Argonauts, Clash of the Titans, and Troy than they can believe in Thor.