Why? You only need licenses per physical CPU. That is a big cost saving.
More info here:
http://www.microsoft.com/sql/howtobuy/multicore.mspx
Specifically:
Microsoft has been driving thought leadership in this area by charging the same amount per processor, regardless of how many cores are in the processor. Microsoft was the first database vendor to make this announcement, in October of 2004, and continues to be the only vendor to date that has taken this position. This strategy is based on the belief that multicore processors are a natural extension of Moore's Law (that the number of transistors on a chip doubles about every one to two years), and that the benefits should be passed on directly to customers.
