How about this?
After the credits, it fades up to 'Thunderbolt' Ross standing in some sort of military control room. He's holding a phone up to his ear:
ROSS: Hello, Col. Fury? It's Ross. Give Mr. Stark a call. We've found him.
As he delivers the last part of the line, he steps to the side and the camera zooms in on a screen that he was blocking, and on it is Banner's cabin.
Just an idea.
Additionally, I've got one for the end of Captain America.
A group of men dressed in heavy parkas with balaclavas covering their faces are scrounging around in the snow for something. It's pretty evident that they're combing the area for something. Suddenly, the two closest to the camera exchange a look and one pulls his balaclava and hood down, revealing that it's Agent Coulson from Iron Man, and he turns to the other agents/pulls out a phone to call Nick Fury, and says something along the lines of "We've got him, sir!" Then (like someone else mentioned), show Cap's old shield under the ice and snow.
Again, that one would be better for the end of Captain America, but it could work as a post-credits scene for Iron Man 2, or even as an opening to The Avengers.