I'd take this with a grain of salt, but somebody is claiming to have seen some scenes from the movie that were screened as part of a toy fair in Asia. The scenes supposedly involve Scarlett Johansson's Black Widow being held captive by some Russian thugs while on an undercover mission. While the thugs are trying and failing to intimidate the captured super-agent, her cell phone rings and she orders the thugs to let her take the call. It's from Agent Coulson, who tells her that Nick Fury needs her to return immediately, at which point she nonchalantly beats the crap out of the thugs while still tied to her chair, which she then breaks and escapes. Assuming that's a legit description — and yeah, that's a pretty big assumption — then that sounds like the character's initial introduction, and we might except similar vignettes for at least some of the other Avengers.
Other scenes shown reportedly involved the introduction of the S.H.I.E.L.D. Helicarrier, in which it first appears to be an ordinary aircraft carrier before rising out of the ocean into the air. Finally, there's a scene of the Hulk being ordered by Captain America to, "Hulk smash!", at which point, well, Hulk smashes. The source also claims it was revealed that the Hulk is more conscious and intelligent than he has been in the previous films. So, while these all sound vaguely plausible - it's not like they're claiming a joint Spider-Man/Wolverine cameo or something - I'd still take these with a grain of salt until we have something a bit more official to go on. [Comic Book Movie]