http://www.sqlservercentral.com/blogs/databaseexpertisecom/2010/08/31/determining-the-no-of-processor-cores-in-your-sql-server/

Determining the no of processor cores in your SQL Server

By Justin H-Davies, 2010/08/31

SELECT cpu_count/hyperthread_ratio AS cores

   FROM sys.dm_os_sys_info;