Robert Pattinson - he's everywhere at the moment because of the Twilight films and regardless of whether people think he's a good actor or not, when they finish filming the series I think he should wait until the hype dies down until he starts filming other stuff as people are still going to see him as 'that vampire guy' and might not take him seriously.
Same goes for Daniel Radcliffe and the rest of the young Harry Potter cast but as Dan has already signed up for a few projects and Rupert Grint has done a couple of small films in between filming Harry Potter, this doesn't seem likely.
I think if any actor is pushed into such a famous role like Edward Cullen and Harry Potter, there would be a great risk of being typecast.