By all counts, the departure of Dag Kittlaus was an amicable parting and long expected. Kittlaus stuck around through the launch of the iPhone 4S, which was all about the integration of the product he started. All Things D says that he wanted to be with his family in Chicago and work on new start up projects. I've never heard of anyone actually believing that "wants to spend more time with his family" is the truth but hey, first time for everything. It'll be interesting to see what he does next.
Also, this news makes me realize how baked into the cultural consciousness Siri already is. It's still incredibly early in the history of AI phone interface but it's clear to me that something new has begun here.