Yes with all versions of ESX from 4.0 upwards you can.Firstly it depends upon version, on how you have to do this,
with version 4.x you have to manaully edit the VMX file
with version 5.x you can set the core count from the settings form.
as to why you would do this
it can reduce license count on those applications that are licensed on Sockets
Tom Howarth VCP / VCAP / vExpert
VMware Communities User Moderator
Blog: http://www.planetvm.net
Contributing author on VMware vSphere and Virtual Infrastructure Security: Securing ESX and the Virtual Environment
Contributing author on VCP VMware Certified Professional on VSphere 4 Study Guide: Exam VCP-410
