You can find a very comprehensive document on licensing TFS 2010 from Microsoft) at the following location
See page 15-20 for details on how TFS is licensed. You get 5 CALs out of the box which is a good deal for smaller development teams - plus no CALs are required for basic use of the worklists. It also details the User Vs Device CAL conditions, and how the External Connector License works.
Note that you must get CALs for "internal" users - internal users cannot use the External connector license. The definition of External users is as follows (though this definition is somewhat vague and open to interpretation):
"External users are defined as users that are not employees of the organization or its affiliates, nor are they employees of the organization’s or its affiliates’ onsite contractors or agents."