May 6's Thor; his hammer, Mjolnir, appeared in the post-credits scene of 2010's Iron Man 2
After starting a war in Asgard, the god of thunder is cast out by his father, Odin (Anthony Hopkins) and sent to Earth, where he plays kissyface with Natalie Portman and fights the Destroyer, a metallic giant sent by his mischievous brother, Loki (Tom Hiddleston).
It was revealed early last year (via leaked details of the movie) that Thor takes place after the events of The Incredible Hulk, which means it's also set after the Iron Man movies.
Marvel's start of production announcement confirmed that Hiddleston would return, so The Avengers will be a personal, family affair for Thor. Hemsworth has revealed that there will be plenty of tension between the Asgardian and the rest of the team, perhaps the Hulk particularly (as has often been the case in the comic books).
He recently told IGN that he's "incorporating some sort of broadsword techniques" for his hammer swinging in The Avengers, and is already "back in the gym" training for the movie. The Thor press tour will keep Hemsworth from the first two weeks of shooting, which may be why he's messed up a few details. Earlier this month, Hemsworth revealed that Portman will appear in The Avengers, a statement that Portman quickly refuted.