SQL Server 2008 R2 Licensing:
“If you license all of the physical processors on the server (one license per physical processor), you may run unlimited instances of the SQL Server software in the following number of OSEs (either physical or virtual):”
SQL Server 2008 R2 Datacenter -> Unlimited
SQL Server 2008 R2 Enterprise -> Up to 4 per license. (Each Server License for SQL Server 2008 R2 Enterprise permits you to run the software in up to four (4) OSEs (physical and/or virtual)

SQL Server 2008 (without R2) Licensing:
“Each physical server licensed for SQL Server Enterprise under the Server/CAL model is also licensed to run SQL Server Enterprise in as many VMs as the hardware will support. So there is no difference between licensing Enterprise edition for physical and virtual environments.”
SQL Server 2008 Enterprise -> Unlimited

So, there is a difference between SQL Server 2008 and SQL Server 2008 R2 but it’s clear that if you license all of the physical processors for SQL Server 2008 R2 Datacenter or SQL Server 2008 Enterprise, you can run any instance of SQL Server 2008 R2 or SQL Server 2008 in any number of virtual operating system environments without regard to the number of physical and virtual processors used.