It looks as though the casting decision has been made and it is Australian actor Chris Hemsworth, of Star Trek, who will be playing the Asgardian Prince Thor.
You may recognize Hemsworth from the opening (and probably most memorable) scene of the new Star Trek where he played George Kirk, the father of James T. Kirk. He is a relatively unknown but seems to be impressing in Hollywood at the moment as he has also just landed the lead role in a remake of Red Dawn.
Being cast as his half brother and God of Mischief is British actor Tom Hiddleston. Hiddleston is another actor, that will be new to American audiences but he has worked with director Kenneth Branagh on the TV mini-series Waalander and the HBO movie Conspiracy, so I am sure he knows what he is capable of.
Hopefully, we will get to find out if there is any more casting decision made plus any more news as time progresses.