When Licensed per Processor. Workgroup, Standard, and Enterprise editions of SQL Server 2005 allow for unlimited instances in each virtual or physical operating environment. For Workgroup, Standard and Enterprise Edition, each virtual operating environment running SQL Server 2005 must have a processor license for each processor that the virtual machine accesses. If a copy of SQL Server is running on a physical operating environment, processor licenses are required for all of the processors on that physical server. For Enterprise Edition there is an added option: if all processors in a machine have been licensed, then the customer may run unlimited instances of SQL server 2005 on an unlimited number of virtual operating environments on that same machine.
SQL per processor can be the solution in a host to run multiple SQL licenses if they are per processor
the following text from http://www.microsoft.com/sqlserver/2005/en/us/Special-Considerations.aspx says: