Heat Vision has just revealed that Idris Elba will play Heimdall, the guardian of Asgard.
Heimdall is the all-seeing and all-knowing brother of Sif (portrayed on film by Jaimie Alexander). He stands on the rainbow bridge, Bifrost, watching for any attacks on Asgard.
MPP - Now, I'm ok with updating roles to fit certain aspects of a movie, but I don't see much resemblance between Idris and Jaimie. If this information is true, and they will portray brother and sister, I'm hoping for a bit of an explanation or re-tooling from Kenny B.
UPDATE: Marvel confirmed the casting of Idris Elba as Heimdall on November 20th.