I think he looks a bit too young and skinny for Ian. I picture someone more muscular like Tatum Channing. I think Liam Hemsworth would be good as Ian. He has acting brothers that could play Kyle too. I'm not sure about Saoirse Ronan as Melanie though. Taylor Kitsch as Jared could work but these are different discussions maybe??
I'm stuck picking between two actors in my mind. Hugh Dancy from Confessions of a Shopaholic and more recently The Big C! I think he would be greaaat! The other actor is Henry Cavill from the TV Show "The Tudors" and movie "Whatever Works" - He's first of all gorgeous! And he is such a talented actor! He can easily portrait all the traits of Ian in my opinion. :D
When I read the host I imagened Ian with big mustles &dirty blonde hair(: