I would be seriously disapointed to see Cap as anything other than an agile, well muscled caucasion with short blonde hair. He's been an icon for what, fifty years? And has looked pretty much exactly the same in every rendition over that time period. If they want to make Nick Fury the Ultimates version, that's one thing, but I think having "the look" of the character is just as important as being a good actor. After the spot on rendition of Robert Downey Jr. as Tony Stark, I can't see Marvel moving away from this trend.
As for who it should be - I'm sure there are lots of great actors that would fit the look for Thor or Cap. I know a lot of people don't like him, but I think Casper Van Dien would make a super Cap. I'd rather see him or some lesser known or unknown actor than someone like a Brad Pitt.