I have used XP and Vista OEM installs on ESXi, without issue. Technically, if your OEM software was tied to hardware that is no longer functional, I believe the EULA states that the OEM copy is to be destroyed, as it's legally tied to the hardware that it came with.
The reason I ask is the XP OEM is to be tied to hardware... with ESXi its not really tied to hardware at all and can be moved around differnt system etc.
Your most likely correct on the oem. Thats why they have excpetions in there DataCenter and Enterprise licensing for servers. I can contact my MS rep if you really want a real answer.