Ex-Xbox chief Don Mattrick lands at Zynga, officially (update: comments from Ballmer and Mattrick)

We just got the news that ex-head of Xbox, Don Mattrick, was out at Microsoft, and now we know where he's going: Zynga. AllThingsD reports that Mattrick is officially the new CEO of the casual social gaming firm, and Mark Pincus, the current CEO, is stepping down from that position to make room for Mattrick's arrival. Fear not for Pincus, however, as the founder of Zynga will maintain a role at the company as chairman and chief product officer. Naturally, Zynga's thrilled with the hire, though we're still waiting word from Redmond on the matter. You can get Pincus' take on Mattrick at the source below.
Update: Steve Ballmer has posted a memo about Mattrick's departure, calling it "a great opportunity for Don" and thanking him for his hard work at Microsoft.
Update 2: AllThingsD has Mattrick's first letter to Zynga's employees, in which he states the "mission to connect the world through games is just getting started."