The idea behind doing Thor's movie is that they want to do an Avenger's movie, and they want each hero to have their own flick before doing the movie of the whole group. That way, less time needs to be spent on backstory (in theory, anyway).
They are laying groundwork in other ways, too... Nick Fury is getting a movie (as well as a cameo in Iron man). Hulk is still going strong ("Hulk strongest there is!"... sorry, couldn't help myself). Captain America is getting his movie after Hulk 2. (which I believe keeps going through script re-writes, or was before the writer's strike), and "Antman" (in planning stages).
My major reservation with Thor is that his backstory requires the most suspension of belief, unless they go the
"Ultimates" route, and you are kept wondering if he really is a divine being, or just a kook with high tech gadgets. Plus, a living god as a hero will get the holy rollers in a tizzy; then again, any publicity is good publicity.
With the writers strike, however, who knows how schedules will change.
Article from August discussing upcoming Marvel movie plans...(Don't even get me started talking about the Justice League movie...)