licensing - How are SQL Server CALs counted?

Running a SQL Server, as far as I understand it, you need one CAL for every user who connects to the database server.

But what happens if the only computer which is accessing the SQL Server is the server running your business layer?
If, for example, you got 1 SQL Server and 1 Business logic server, and 100 Clients who all just query and use the business logic server. No client is using the SQL Server directly, no one is even allowed to contact it.
So, since there is only one computer using the SQL server, do I need only 1 CAL???

I somehow can't believe this would count as only 1 CAL needed for the SQL Server, but I would like to know why not.

Answer

You need CAL's for every user of the business logic server, even though there is no direct connection between them and the SQL server. Microsoft use the term "multiplexing" for the scenario you describe.

This is for SQL 2005 but I don't think it is any different for other versions:

A CAL is required for each distinct device or user to the multiplexing or pooling software or hardware front end. This remains true no matter how many tiers of hardware or software exist between the server running SQL Server and the client devices that ultimately use its data, services, or functionality. An exception to this includes the manual transfer of data from employee to employee. For example, if an employee sends a Microsoft Excel version of a report to another employee, the receiving employee does not require a CAL. An additional exception is communication exclusively between the servers running SQL Server.

Anyone using your Application has to be covered by a CAL, including someone who uses it solely to generate Excel Reports but someone who only receives a copy of one of those Excel Reports by e-mail doesn't.

Edited to add This is just one of many aspects of SQL server licensing and it's only relevant if you choose the Server+CAL licensing model. If you want to get a full understanding of all of the alternatives the definitive document is Microsoft's 63 page SQL Server 2008 Licensing Guide which covers all licensing models and approaches.