Yep, he has already been interviewed in which he explains why he decided not to direct Iron Man 3. I thought this deserved it's own thread since his departure isn't news any more, but why he left (in his own words) is. And it is apparently not about money or creative control:

In an interview with L.A. Times writer Geoff Boucher, Favreau said the rumored reasons why he’s leaving Iron Man 3, such as money issues or lack of cohesiveness, were inaccurate. He’s still a producer on The Avengers and remains friendly with Marvel main man Kevin Feige. His main reason for leaving was to “find something that lights a fire” inside of him and also something that will “blow people away, which is easier to do with a project that isn’t loaded with built-in expectations.” So, basically, he wants some new toys to play with. He thinks of the departure as more of a “graduation” rather than a “divorce.”   http://www.slashfilm.com/jon-favreau-explains-decision-direct-iron-man-3/