Latino Review are reporting that Tim Robbins is set to play Howard Stark whilst characters Hawkeye and Black Widow are rumored to make an appearance in the Iron Man sequel:

From what I’m told, Tim Robbins is going to be playing Howard Stark in an important flashback sequence that is going to set up the Avengers and Captain America films. Jon [Favreau] wanted to get a good actor because he didn’t think the guy who played Howard in the last film could carry such an important scene.

The scooper goes on,

I’m also told that HAWKEYE and BLACK WIDOW are definitely in Iron Man 2 and they wont be cameos either.