May 6's Thor
Up until Marvel announced that Hiddleston would return in The Avengers, Loki was just a rumored possibility, stemming from the character's inclusion in the first issue of the Avengers comic book. However, unlike the comic book, Odin's other son may be up to more than just controlling the Hulk's mind and unleashing him on the public.
As Thor director Kenneth Branagh said in February that "a brother who is unhappy is a dangerous relative to have." In March, an online rumor had Loki gaining possession of the Cosmic Cube, and using it to bring the alien race of Skrulls to Earth to fight the Avengers, but this is unconfirmed.
Another Loki rumor comes from a proposed post-credits scene in Thor, which we don't want to spoil, but, apparently, involves the character of Professor Selvig (Stellan Skarsgård, who has also been confirmed to return to The Avengers).
For those not interested in spoilers, Hiddleston recently told CinemaBlend that Loki will be "not so concerned with beating his brother anymore" in The Avengers, and while he wouldn't elaborate on Loki's plans, he did reveal that "because his plans get bigger, the necessaries needed to defeat him, I think, need to be bigger."
Up until his confirmation, all Hiddleston has done is deny involvement in the movie, or, at least, pretend to not know anything about his involvement. You can't blame him, Loki's inclusion in The Avengers is obviously rife with secrets, so Hiddleston couldn't talk about his role even if he wanted to do so.