A message board post at Superhero Hype, confirmed by a number of other sites, there has indeed been a second post-credits scene added, in accordance with Robert Downey, Jr.'s claims the night of The Avengers' world premiere in Los Angeles that the cast had reassembled to film a little something extra.
The scene, which has not yet been screened overseas and may be exclusive to the American audiences, shows The Avengers, apparently immediately post-battle, sitting in a restaurant together eating shwarma. It's a callback to an earlier scene wherein Iron Man--having just suffered major battle damage and giving everyone a bit of a scare--opens his faceplate and asks if anyone wants to try schwarma at a restaurant a couple of blocks away.
When the scene is supposed to take place is a question, of course--Tony doesn't have his armor on and Banner is fully clothed, suggesting that the team must have had time to go home after the big battle...but the presence of Thor seems to fly in the face of this scene being chronological with the rest of the movie.
It's also questionable whether the film was actually filmed when they said it was or whether it already existed and was simply added later; Chris Evans' beard does not appear in the scene. Of course, if it did, that could have created a whole other continuity nightmare, so that's probably for the better.