Well, basically because he is in the New Avengers. They added him as an actual Avenger about two years ago with Iron Man, Thor, Hawkeye, Captain America, Wolverine, and Spider-Woman, but he and Wolverine went back to focusing on New Avengers after the Fear Itself event last year. Otherwise, Spider-Man doesn't have much of a history as an Avenger.
Me too. Casting will be a big part of this movie though. Marvel has a pretty good track record so far. I know others have complained about Thor and Captain America, but I think they were strong first movies. If you ask me, the only blemish on Phase One was Iron Man 2.