Latino Review is reporting that one of their sources is saying that Tim Robbins has been cast as Howard Stark in Iron Man 2. Besides being Tony Stark's father, it is being implied that he will play an important role in the formation of the Avengers. According to them:

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 speculation is that in the film, Howard Stark, via flashback, will somehow be involved in the Super Soldier Experiment, which is the catalyst for the origin of none other than Captain America. Additionally, they are also reporting that Avengers team members Hawkeye and Black Widow will be in the film. (And not just as a small cameo.)

Reporting rumors is what we do here, so take it for what it is worth. However, if indeed this passes the B.S. detector, this has to be considered good news. One of the most promising aspects of the upcoming Marvel Avengers-themed films, is the promise of a singular, coherent universe between the films. It has never been attempted before, as filmmakers always have given in to the creative vanity of telling a consolidated story within their films (often resulting in the deaths of key characters.) The walk-on cameo of Tony Stark in The Incredible Hulk was just the jolt that not only that film needed, but sent all the right messages. So, if this is the direction towards which we are heading, then I say "floor it."

Source