This has been rumored for some time now, but it now appears to be coming from some official news sources (like Reuters).

PIXAR’s Brad Bird will direct Tom Cruise’s next Impossible Mission err Mission Impossible.
Bird’s no stranger to action flicks, like PIXAR’s 2004 “The Incredibles” but this does mark a new adventure for him as he directs his fist live action feature film.

IMDB finally put up a page for MI4 today and all they have confirmed so far is that Cruise will be reprising the role of “Ethan Hunt” and that Bird is directing.  The story for MI4 is being penned by JJ Abrams (ABC’s “Lost”).