Many felt it was inevitable that Tom Hiddleston's Thor villain Loki would also pop up in Joss Whedon's Avengers to trouble our mighty heroes. But then everything seemed to be up in the air. Hiddleston wasn't sure, the Skrulls were(and still are) rumored, and then word seeped through that the God Of Lies wouldn't be making an appearance at all.
Now io9 have received the following info from a scooper..
The post-credits scene makes it pretty clear that Loki WILL appear in The Avengers. I think it's worth looking out for Stellan Skarsgaard on the cast list...!
They say they can't confirm it, but we already heard from Skarsgaard that he will most likely be in
The Avengers, so perhaps the post credits scene also confirms Loki. Guess we will have to wait and see the movie to be sure. But of course even if he does APPEAR in the movie, that doesn't mean that he will be the main villain, or even pose much of a the rest, it may just be a cameo relating to Thor and not the rest of the team.