Licensing

Visual Studio Team System 2008 Licensing White Paper
http://www.microsoft.com/downloads/details.aspx?FamilyId=CE194742-A6E8-4126-AA30-5C4E969AF2A3&displaylang=en

Summary
A CAL is required for each user that accesses TFS, via any client. The only exception on this is the Work Item Web Access. A CAL is included in any Team Edition (Architecture Edition, Developer Edition, Test Edition or Team Suite). In any other case you have to buy additional CAL's.