As an evangelist for Visual Studio Team System one of the most common questions I get from customers evaluating our tools is, "Does Microsoft use Visual Studio Team System?" Until recently, my answer was always along the lines of "Well, a LOT of teams at Microsoft are using it with great success. Including nearly 1000 people in the Developer Division. But as with any technology migration, it will take a while for huge teams working on huge software projects like Office and Windows to begin to adopt Team Foundation Server."
But today I learned that the Windows division has started to adopt Team Foundation Server! Some of the feature teams in the Windows division have begun to make use of work items for tracking features and tasks. This is just the first step along the way towards a wholesale migration of Team Foundation Server by the Windows team. Additionally, I'm not sure if anybody else has blogged this yet, but the Office team has also been using Team Foundation Server for work item tracking as well and is on their way towards adopting more and more functionality along the way.
A big part of the reason these teams are betting on Team Foundation Server is because of the way it has helped other teams at Microsoft gain transparency into the health and velocity of their software projects. By taking all of the artifacts of a software projects (requirements, tasks, source code, bugs, etc.) and linking these together and storing them in a data warehouse you can really begin to generate compelling reports which allow for a very high level of transparency. If all of this sounds interesting to you, be sure to read Sam's book.