It seems like he didn't really quit either but was let go gracefully. As head of iPod division it seemed as though he was more protective of the iPod's role as Apple's superstar product then of the big picture Jobs wanted to push. Rumor was he wanted to use the iPod OS for the phone and then he gave an interview about how converged devices were lame and 3 weeks later they announced his "reduced role" as a consultant and shortly after that he was out.

Granted that's mostly based on rumors but it gives an interesting contrast to Microsoft's failure in tablets because of things like the Office division's execs refusing to make it touch friendly.