Brian Harry has posted an article on how Microsoft has started eating their own dog food by implementing Team Foundation Server within their own ranks. He details several of the issues they have ran into so far - most of which are related to the sheer size of their various projects. For example, one of the statistics he posts is the number of files and folders currently in their TFS system - 13,137,178 files and 1,677,668 folders. That's a lot of files although, according to his post, it will be a lot more than that by the time they're through.
Brian also goes on to state that they've fixed several bugs and performance issues internally within Team Foundation Server that they will be making publicly available (presumably sometime later this year).
It is a very interesting read to see how Microsoft is making use of their own tools.