Matt Gertz, Developer Division's manager responsible for our internal development tactics and process, recently posted an article covering the broad post-VS2008 efforts to more broadly adopt TFS within DevDiv for future product development. As regular readers of my blog know, Team System teams have been dogfooding TFS for a long time now so this change is about getting the rest of the division switched over as well. We've certainly had our share of growing pains (ever try unshelving a shelveset with 1 million files?) but we're pretty much through the transition (and fixed a number of scale related issues along the way). I posted a recent report on internal usage stats for TFS and in it you can see the increase of DevDiv users from 1,300 to 2,200 since December '07. Now you know what prompted that considerable increase.
If you're interested in learning more about DevDiv's dogfooding of TFS, be sure to read Matt's article: http://blogs.msdn.com/vbteam/archive/2008/03/28/milestone-quality-dogfooding-matt-gertz.aspx.