I've come across a few articles about Pixar employees who have left the company to direct their own animated/live action films, and have found it incredibly interesting. It has also somewhat changed my perspective on the studio...
The first, and the one I can most relate to, is Jan Pinkava.
Jan moved to England when he was young, but he was originally born in Czechoslovakia. He made short animations when he was young, winning awards for them at local film festivals. He then studied Computer Science following with a PhD in Theoretical Robotics, and travelled to London to direct commercials in London at a company called Digital Pictures. After being inspired by Luxo Jr. he quit and freelanced for a while, before sending his resume to Pixar.
There he worked as a director for the commercials that the studio was outputting whilst working on Toy Story, and then got the chance to direct Geri's Game, a short film that won the Academy Award in 1997.
After these this early success, Jan spent his time animating and storyboarding on various Pixar films, before giving the go ahead to direct his first feature. Jan came up with 3 treatments, and the one that received the best reaction was Ratatouille. He worked as director for a good 3-4 years on the project before he got replaced by Brad Bird. I don't know the full story but I know that the film went into development trouble and that another director was needed in order to get it back on track. Jan became co-director and went on to finish the film with the rest of the production team, but left Pixar shortly after production on the film ended.
I am somewhat surprised to hear this particular story, and my feelings towards the matter are a little mixed. On the one hand, I understand Pixar's concern that if there is a problem with the film they should do everything in their power to rescue it and bring it to a high standard. On the other hand, I can't help but empathise with Jan's position of leading the helm for a large duration of the film, only to be usurped by another director (one already boasting in an academy award for animated features). I know in his position I would have been incredibly demoralized. However, it IS still the film he intended to make, and he has made the necessary steps in order to proceed in making his own animated features still.
There are two other employees that caught my attention. Jimmy Hayward and Ash Brannon.
These two followed similar paths to Jan, in the sense that they both had to leave the company to pursue directing. Jimmy worked his way around the departments at Pixar, eventually deciding to leave and finding a haven in Blue Sky studios, where he directed Horton Hears a Who. He's no helming the director's role on the forthcoming live action film Jonah Hex.
Ash Brannon was the initial director of Toy Story 2, but once it was decided that it was to be a theatrical release than a straight to dvd film, John Lasseter stepped back into the director's chair and took over the film. Ash later left after his attempt to pitch internet short films wasn't successful, and eventually wound up at Sony where he directed Surf's Up.