Are you using vSphere Converter? What are the guest OS, are they Win7 or Server2008. I've done several P2V with XP and Server2003, did not see any licensing issues with OS.
In reply to Travis Neale:
That makes sense, AFAIK OEM licenses are not portable. The Servers and OIT I converted were most probably built or rebuilt at some point in time using Emerson's S2003 and XP Volume ID. Boot from CD or ISO and do a repair, after repair it will boot up and ask for a key. Set the correct ID and you should be all set.
As an aside, when you create a new virtual machine it creates a new hexadecimal identifier for the emulated motherboard and new MAC addresses for your emulated NICs. A change in either of these on a physical or virtual machine can trigger the reactivation of Windows. The Dell OEM licenses are tied to specific hardware, so the Windows activation and key authentication on the virtual machine will fail if you use the Dell OEM key. Similarly, if you built a whitebox PC and tried to use a Dell OEM Windows license, the authentication would fail. Windows senses the change in underlying hardware and triggers reauthentication. It would trigger if you loaded the OS instance in a virtual machine or if you loaded it on new physical hardware.