In a recent Variety article which can be found here, it is confirmed that Guy Pearce is in final talks to play geneticist Aldrich Killian.
The Aussie actor is the lead in the newly released 'Lockout' and after that appears in Ridley Scott's 'Prometheus' and John Hillcoat's 'Lawless', premiering at Cannes.
The plot of Shane Black's Iron Man 3, which is based on Warren Ellis's Iron Man: Extremis comics, will also see the return of Don Cheadle and Gwyneth Paltrow's characters, and Ben Kingsley is said to be playing the villain. For those of you who don't mind some minor spoilers - Killian is the character who creates Extremis technology and sells it to terrorists - before Tony Stark adopts his own version of it.
The production of 'Iron Man 3' begins in May.