However, Kingsley's role won't be the Mandarin, according to Variety, but something more related to Warren Ellis' "Extremis" arc from the Iron Man Marvel comic books; Variety states Kingsley's role will be involved with spreading the Extremis virus.
That six issue series was also the basis for John Favreau's new Iron Man origin, as well as the introduction of nanotechnology, and Favreau even brought in artist Adi Granov as a producer.
The Extremis "virus" is basically nanotechnology that re-writes the human body into something strong, better and more powerful. However, it can only be used in a handful of the population, with the rest getting terminated by the virus. It remains to be seen how much of Ellis's arc will be adapted, as Tony used the Extremis on himself.
There is a motion-comic on Netflix, which you can check out as well.
Iron Man 3 blasts off May 3rd, 2013 directed by Shane Black and starring Robert Downey Jr., Scarlett Johansson (Black Widow), Gwyneth Paltrow and Don Cheadle.