We've actually got quite a number of case studies posted on MSDN, but some of them are a bit difficult to find due to the way the documents are currently flagged. We're fixing that, but in the meanwhile here's a cheat sheet listing the ones I know about, the specific features they use, and what product you can find those features in...
The customers listed here were using pre-RTM builds which had all VSTS functionality. We didn't split up the components into the role based products till RTM, so these customers effectively had VSTS Suite available to them.
TFS = Team Foundation Server
VSTE = Visual Studio Team Edition
WI= Work Item Tracking