With the release of vSphere 4.1, VMware has introduced Multicore Virtual CPU technology to its bare metal flagship hypervisor.  This is an interesting feature which had already existed in current versions of VMware Workstation.  VMware has consistently baked in new features in its Type 2 hypervisor products, such as Workstation, Player, Fusion, etc., more or less as a functionality/stability test before releasing the same features in ESX(i).

Head on over to the link below for the excellent write up by Jason Boche’s Blog for all the details, including how to setup the multiple cores within ESX(i)!

http://www.boche.net/blog/index.php/2010/07/25/vsphere-4-1-multicore-virtual-cpus/

****Update 8/31/10****

The article above is good however it doesn’t really mention the requirement that you have to allocate more vCPU’s to the VM in ADDITION to making the cores change like the article states. VMware’s KB article is a little clearer in this area, including the math needed with allocating vCPU’s and Cores. The KB article is at:
http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&externalId=1010184&sliceId=1&docTypeID=DT_KB_1_1&dialogID=109091853&stateId=0%200%20109093171