Casting is key with Capt America. He's the leader of the Avengers and he has to command respect on screen when he's surrounded by other respectable actors (Downey Jr, Norton). Capt is Blond haired blue eyed and has the morality of Superman. Who do you think would make a great Steve Rogers AKA Captain America?
My pick would be:
Daniel Craig. He can do an American accent, and he's definitely made a name for himself as Bond, and he still shouldn't be too cost prohibitive yet.
Some other Blond actors:
Brad Pitt
Paul Walker
Kiefer Sutherland
Leonardo DiCaprio
Jude Law
Matt Damon
Josh Lucas
Matthew McConaughey
Mark Wahlberg
Thomas Jane