The folks at Warner Bros. are making a King Arthur film with Guy Ritchie directing, and have already set a release date of summer 2016. They've also zoned in on a star name, but not for the titular part.
According to Variety, Idris Elba will be offered a major role in the film, a "Merlin character" who teaches Arthur stuff. We're assuming it's mystical, medieval stuff, not birds and the bees stuff. That would be a cool Judd Apatow version with Seth Rogen as Arthur, though. Eh hem, anyway...
The role of Arthur is yet to be cast and Ritchie and the studio are apparently testing people in the next few weeks for the part, but no names have been mentioned.
The last King Arthur film was actually shot in Ireland in 2004 and starred Clive Owen.
Ritchie has The Man From UNCLE due next, with Henry Cavill and Armie Hammer.