How is the license usage of individual VMs determined in the server/CAL model of “SQL Server 2008”?

For „SQL Server Standard“ and „SQL Server Workgroup“ the following applies:
Every virtual machine with SQL Server is considered as a physical machine and therefore must be licensed with a separate server license.
For „SQL Server Enterprise“ the following is valid:
All physical servers on which the virtual machines can be operated must be licensed. The number of VMs is not important.

Source: sqlserver2008_licensingguide.pdf