While no plot has been leaked or alluded to for The Avengers the common assumption is that the story would follow the one that brought the team together in the first place. That being that Loki, to debut in the Thor movie, manages to manipulate the Hulk into doing his bidding by some means.
The first step is confirmed, that Loki will appear
. Now we just need to know for sure if the story follows that he uses the Hulk like a pawn.
...the site mentions that Tom Hiddleston will return to play Loki. Ok, it's not a confirmation that Loki will brainwash anyone, but it's something.
With all this casting news it seems clear the production is still on track for Spring. How cool would it be to get some type of teaser imagery during the Captain America movie!!