Thor grossed $437 million worldwide, and Marvel Studios has a public mandate to release two movies a year. 

In 2011 it was Thor and the upcoming Captain America: The First Avenger

In 2013 it will be the superhero team-up The Avengers with no second release yet announced.

In 2013 we’ll get Iron Man 3 on May 3rd and Thor 2 later that Summer.

Walt Disney Pictures confirms.
The Avengers 2 is in the works as well as a Nick Fury film. 

At the moment we have the following either released, pre-production, or planned:

The Incredible Hulk
Iron Man
Iron Man 2
Iron Man 3
Thor 2
Captain America
Captain America 2
The Avengers
The Avengers 2
Nick Fury

There are also talks about a HawkEye, Hulk 2, and some other minor character spinoffs. 

Screen Junkies got a chance to talk to Captain America: The First Avenger co-writer Stephen McFeely about a possible sequel. Here are some highlights from the conversation:

Q: We know he comes into modern day for The Avengers, but is there room to go back to WWII?

SM: Well, our hope is that there is room and we’re negotiating how to do that now. The story will likely be in the present day. We’re experimenting with flashback elements for more period World War II stuff. I can’t say much more than that but we made it baggy enough to refer to more stories in the past.

Q: So you left gaps in Captain America 1 where you can always say something else was going on back then?

SM: Absolutely. He had more adventures than just the one you’re going to see in the movie.

Q: Do you know who the villain of Captain America 2 would be?

SM: It’s undetermined. I will pass that question, how about that?

Q: When would you be scheduled to work on a sequel?

SM: We’ve already made the deal so I was at Marvel last week. We’re talking and passing stuff back and forth all the time. They just sent me a big PDF file of comics.