Preeminent superhero movie lexicon, superherohype.com has a bunch of amazing shots from the set of the Avengers. Marvel's big boys hit theaters next summer with an as yet unknown villain opposing them.
The speculation is that Loki will be the main threat, something that fits with Marvel Comics lore- he was the first baddie they faced in the comics after all. But take a look at the pics and tell me if you don't get the feeling that there's something else going on too.
Maybe something a little more... alien?