As far as I researched about this matter, MS don't care what virtualisation software that is serving your virtual Windows server (it could be ESX, Hyper-V, Xen and so on).
A change from 2003 to 2008, is that in the Standard Edition, MS allows you to 1 physical and 1 virtual instance on the same license. On the Enterprise and Datacenter editions, the rules are the same (4 virtual + 1 physical for the Enterprise, unlimited virtual + 1 physical for the Datacenter). Datacenter is licenced under number of CPU's though.