The problem I came across with small business server is the CALS are so damn expensive in comparison to Server 2003. I was going to have it set as my main domain server, but the cost for CALS per user; per device or per server was just too high. It was much cheaper for me to get Server2003. I do want to use Exchange though and have been looking into SBS. Now that I have a domian controller, I could add SBS on the network and only purchase CALs for the workstations actually using it's services.
You only use one licensing model...

You do not license per user, AND per device and per server.

users can be devices.devices can be servers and vice versa.........the SBS just looks at concurrent connections for licensing....you can have 200 users...only licensed for 20...as long as they dont all connect at the same time.....only 20 users\devices\servers will be able to connect

and if you add another server...it is either a user or device on your network...and its resources as a basic server are licensed under the SBS...unless you add an application to that extra server...then you will need licensing for the app.....

MS is very good at answering licensing questions..just layout what you want to do...and they will tell you what you need to buy.

MLF