I thought Hemsworth was great in the small role he was given in Star Trek. He definitely made that opening scene emotionally powerful, while still coming off as a strong character. That whole scene kind of makes me pine for a new Star Trek series set aboard the Kelvin in the original, unaltered universe. Oh well.
Back on topic, given the small amount of work I've seen him in, I think Hemsworth will be excellent as Thor. My only thought is about how well he'll mesh with RDJ, Jackson and Norton (if he's in it, is that guaranteed?), as well as whoever is cast as Captain America in the Avengers movie. I would hate to see a character who is a god be overshadowed because of the performances and star status of the other characters and their respective actors.