This is probably a FAQ but having read the licensing paper and http://msdn.microsoft.com/vstudio/products/compare/, I'm not 100% sure if I can use work item tracking and version control without a VSTS SKU (in the scenario where developers only version control).

VS 2005 Pro with TFS client and source control
Ronin_KahO
Kataria Bipin
Dennis.Vroegop
rhernandez_64
A TFS CAL allows you to access the server from any client: any version of VS, a 3rd-party app, a web interface, whatever.
Only VSTS SKUs include a CAL in their price. So if you're using VS Pro, you'd have to purchase a CAL separately. (Unless you're using TFS Workgroup Edition, which does not require CALs.)