Updated June 2011: Richard Madden has been in a couple of tv shows in the UK, none of which I've heard of. He's doing well with Robb's character so I'm pleased with this casting.
I imagined Robb as Henry Cavill, who was in The Tudors.
Robb is Eddard Stark's eldest legitimate child, and the heir to Winterfell. He bears great responsibility and shares his father's devotion to honor and justice. His appearance favors his Tully side, with a stocky build, blue eyes, and thick auburn hair. His companion direwolf is named Grey Wind. When Eddard became the King's Hand, Robb remained at Winterfell to rule in his father's stead.
I thought Henry Cavill from Stardust would make a good Robb Stark.