When Kevin Bacon officially signed on to play the villain in X-Men: First Class, Fox did not specify which X-Men rogue the actor will portray.  Despite heavy speculation that Bacon would be perfect for Mister Sinister, Forces of Geek have a source that claims the villain in question is Sebastian Shaw.  Here's a brief character bio courtesy of Wikipedia:

A mutant, Shaw possesses the ability to absorb kinetic energy and transform it into raw strength. He is the leader of the New York branch of the Hellfire Club, an exclusive secret society bent on world domination, although to the public, he is a legitimate businessman and ordinary human.

X-Men: First Class is set to begin production next month with Matthew Vaughn (Kick-Ass) as director.  James McAvoy, Michael Fassbender, Nicholas Hoult, Alice Eve, and Caleb Landry are official members of the cast, while Aaron Johnson, Rosamund Pike, and Lukas Till have been rumored for roles.  Fox is targeting a June 3, 2011 release date.