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.

Company Features used Product
Vendex Application Designer VSTE for Architects
Process, WI, Version Control, Reporting TFS
Imagination Application Designer VSTE for Architects
Team Build, Reporting, Version Control, WI TFS
Fujitsu DSL Toolkit VSTE for Architects
Process, WI,  Reporting, Excel and Project integrations, Project Portal TFS
Barclays WI, Reporting TFS
TCSC WI, Team Build,Reporting TFS
Web Testing VSTE for Testers
Co-eXprise Version Control TFS
Unit Testing, Code analysis, Profiling VSTE for Developers
Infosupport WI, Reporting, Team Build, Process, Project Portal, Version Control TFS
Airways New Zealand Version Control, WI, Team Build, Reporting, Excel and Project integrations, Project Portal TFS
GEICO Code Analysis VSTE for Developers
Worldspan Load Testing VSTE for Testers
  Profiling VSTE for Developers
State of Washington WI, Reporting TFS
HP Process, WI, Reporting, Project Integration TFS
wine.com Unit Testing, Code analysis VSTE for Developers

TFS = Team Foundation Server   

VSTE = Visual Studio Team Edition

WI= Work Item Tracking