Setting processor affinity does not allow you to reduce the number of processors for which you are licensed. Any processor to which the OS has access must be licensed .
Creating a virtual machine with fewer virtual processors than the physical host contains does allow you to reduce the number of processors for which you are licensed. You only have to have licenses for the number of virtual processors .
 If you run the software in a physical OS environment, you need a license for each physical processor used by the physical OS environment.
 If you run the software in virtual OS environments, you need a license for each virtual processor used by those virtual OS environments on a particular server—whether the total number of virtual processors is lesser or greater than the number of physical number of processors in that server.
These two references are both from page 6 from the following document:
MS Forum: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1884303&SiteID=1
MS Licensing FAQ: http://www.microsoft.com/sql/howtobuy/faq.mspx#EZB