He created and led speech recognition for Siri. Kittlaus was actually CEO of Siri since 2007. In 2010, Apple bought the software for an undisclosed sum (although Business Insider estimates around $100-200 million). Along with Kittlaus was Adam Cheyer (VP of Engineering), Tom Gruber (CTO), and Gummi Hafsteinsson (VP of Product), who are not expected to leave Apple.
Dag Kittlaus stated this was a mutual departure, planned before Siri was introduced last month. He wants to spend more time with his family in Chicago. He also wants to start brainstorming new ideas.
Siri, in the meantime, still continues to be a hit on the iPhone. The latest commercials show all the things you can now do with your mobile device. It has spawned a Siri clone (called Iris) on Android within 8 hours.