Brian Harry just sent out the latest statistics for how we're using Team Foundation for our own internal development environment.
The most significant news is that he decided to try and push the limits of the system by adding "tons" of files. In fact, he added another 200,000 files into the system - which represents a very high percentage of the Whidbey codebase. This did uncover a couple of SQL bugs along the way, and doing the massive checkins helped us find another bug which will be fixed the next time we refresh our dogfood servers with new bits.