"The rumors are true. The team is in great hands. I'll miss working on #wpdev. I will the community, but won't be a stranger."
In a way, this should not be a surprise as his old Twitter profile even proclaimed "Entrepreneur on loan to Microsoft focused on getting our developer mojo back." That statement already read to us that this was a temporary job for him with a finite mission, one that looks to have finally succeeded.
No doubt, he'll leave quite a hole to fill as Watson had quite the presence in getting Windows Phone back on the map, working tirelessly to get the recognition it deserved and making devs happy.
Update: Watson is leaving for Amazon. In an email to ZDnet he mentions he'll be taking on the responsibility as Director of the Kindle Cross Platform team and “it was a hard decision, but the opportunity placed in front of me that was too big to pass up.”