Paul Thurrott's Supersite for Windows has posted Steven Sinofsky's full letter to Microsoft employees announcing his departure. Sinofsky says that, following the Windows 8 and Surface launch, he has "decided to leave the company to seek new opportunities that build on these experiences."
Some might notice a bit of chatter speculating about this decision or timing. I can assure you that none could be true as this was a personal and private choice that in no way reflects any speculation or theories one might read-about me, opportunity, the company or its leadership.
This contrasts with what The Verge has heard from sources within Microsoft, which have attributed Sinofsky's exit to an abrasive personality that was seen as an obstacle to collaboration despite the man's undoubted talents. Our sources wouldn't call the departure a firing, but together with earlier reports of Sinofsky's reputation it does seem clear that many at Microsoft will be happy to move forward without him. Time will tell if new Windows head Julie Larson-Green proves easier to work with.