This apparently came to light back on the 3rd but I didn't see it.... Below is the first part of the post on his site.

Over the past year Iíve been doing quite of bit of thinking. Nothing specific, nothing groundbreaking. Just plain old thinking. I turn 40 this year. Despite the wonderful ability in Microsoft to change careers pretty dramatically inside the company, Iíve been there nearly 18 years and itís the only world Iíve really ever known. I feel too strangely comfortable, and too strangely tied.

So over the past several weeks Iíve made the decision to resign and have been working with my management for an orderly transition. My last day will be February 15th.