Out: Conan Stevens.

News has surfaced that the role of Ser Gregor Clegane will be recast for the second season. Conan Stevens, who played the role in two episodes of Season 1, has been working on the second season of Spartacus: Blood and Sand and also on on the movie version of The Hobbit. Initially it appeared that he would be able to do all three projects, but it seems that scheduling conflicts have arisen (possibly sparked by the decision to split filming of the Hobbit movies into two blocs, with the second bloc being pushed back directly opposite filming of Thrones).

Stevens has been replaced in the role by Ian Whyte, who has already appeared on the show as White Walker in the first episode. Whyte, at 7 foot 1, is the same height as Stevens
Ian Whyte

In: Ian Whyte.

and is also an experienced stunt and combat performer. He played most of the Predators seen in the two Alien vs Predator movies, for example. Whyte has already filmed a lot of Gregor's material for the second season.

Sad news, since Conan Stevens campaigned vigorously to get the role of Gregor. On the other hand, he always said that his plan was to use the role as a springboard to greater heights, and certainly working on the most eagerly-awaited movie on the planet at the moment would qualify. Good luck to Conan on his future projects :-)