I think you'll find the general consensus here is don't do it if it's a production server and a DC that has replication partners. You potentially corrupt AD by doing this. It's better to make a new server, dcpromo it and move any FSMO roles to it then shutdown the old one. You can also just dcpromo the original to a member server, migrate it then dcpromo it back in the domain. You may not run into problems cold-cloning it, but if you do AD problems are never fun to resolve. If you must clone it then cold-cloning is the way to go over hot-cloning.
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See these links...
Virutalization of Active Directory - http://www.vmware.com/community/thread.jspa?messageID=352424񖂨
Considerations when hosting Active Directory domain controller in virtual hosting environments - http://support.microsoft.com/kb/888794
Virtualizing a Windows Active Directory Domain Infrastructure - http://download3.vmware.com/vmworld/2006/tac9710.pdf
How to detect and recover from a USN rollback in Windows 2003 Server: http://support.microsoft.com/kb/885875/
How to detect and recover from a USN rollback in Windows 2000 Server: http://support.microsoft.com/kb/875495/
P2Ving Domain Controllers - http://www.vmware.com/community/thread.jspa?messageID=390630