A reliable source tells the IESB that the character of Donald Blake has been added to the end of the script. Let me reiterate that he wasn’t there before, the script was set entirely in the land of the Norse Gods.
The introduction of the character is essential to how Marvel will tie Thor into the Avengers that is set in contemporary Tony Stark/Hulk timeframe. It explains how the actual Thor film will be set in the ancient time of the Norse Gods and how they will subsequently bring the character into a modern time with Donald Blake discovering the magical Norse hammer Mjolnir and turning into Thor.
The site adds that Thor director Kenneth Branagh will have an acting part in the film as well.