MS has a two part license; One for the server and one for the client. The install licenses are attached to the server and the client licenses the user (or device).

To run a SharePoint system you need;

To Install:
Win2K8 license
MOSS license

To Access
Win2K8 Client access license
MOSS Client access license

The user may already have a Win2K8 CAL. But for users outside the Org, such as guests you much purchase both. H

You can also by a connector for an individual server. However, "Per User/Per Device mode tends to be the most economical designation for Windows CALs in distributed computing environments where multiple servers within an organization provide services across most devices or users."

It seems that an External Connector is good for a single server, and a User CAL is good for a user on any server. They seem to cut off at the org level, such that if I have a CAL it's only good for the org that bought it, and I cant use it when I access a server in another org. seems to indicate that a User CAL is good against any (2008) server the Org has
(09:51:48 AM) Im guessing the external connector is cheaper, but that starts to go away when you have more than one server and