Or at least Salon thinks he is. They caught up with Cooper at Cannes, and they mention he's being tipped to play young Howard Stark in Captain America. The wording leaves it unclear whether he's officially signed, or merely in the running. Cooper doesn't even mention it during the course of the interview, but presumably did once the camera was turned off.
Those of you who remember the tiniest details of the casting process may remember that Cooper was up for the part of Fandral, one of the Warriors Three in Thor. He lost that part to Stuart Townsend, who promptly lost the part to Joshua Dallas days before filming began. Could Salon have just been mistaken, and misremembered old Marvel news? Or did Marvel keep his name on a "To Cast In Something" list, as they have with many of their actors and actresses, and offered him the part of the senior Stark? I suspect the latter. I'll be sad to see Slattery go (a little hairdye and make-up, and I think he'd be fine), but Cooper's a good actor, and certainly looks like he could have fathered Robert Downey Jr. We'll know soon, as Captain America goes before cameras next month.