From Irena Kennedy:
SQL Server Enterprise Edition customers, and I quote, “only need to purchase one license per physical processor regardless of the number of virtually deployed instances. “
In other words, “when a customer licenses all of the processors in a server, they can run an unlimited numbers of SQL Server instances on an unlimited number of virtual operating system environments, at no additional licensing cost. “
This is going to be an important step in fostering the virtualization software.