virtualbox.org

Since it's now possible to clone VM's easily (and probably scriptable via clonevm) I need a way to have all my cloned Windows guests have unique/dynamic computer names.

Is there an easy way to do this short of creating a Windows domain controller/dhcp server?

A computer/workgroup/domain name is a config option for software run by the guest OS. "VBoxManage guestcontrol" provides you with a way to run software inside the guest. If you have a guest program that can change domain names etc then that is how you could run it.