In the past I've mentioned slmgr.vbs. I just used it to remotely provide a key to a workstation and activate it. It was very simple. The syntax took me a couple of tries but here it is -
slmgr.vbs machinename domainname\username password -ipk PID
This first step will provide the machine with a key. Substitute your info in all the appropriate places including your product key in place of PID.
slmgr.vbs machinename domainname\username -ato
This step activates the machine.
If the target isn't a member of a domain you can use a local administrator account. You also will need access through the local Vista firewall to make changes in WMI. This is documented in the Volume Activation Step-by-Step Guide which is a must-read for deployment experts. A copy is also provided as a document in the beta download of BDD 2007 on Connect.
These steps appear to work for any version of Vista. I have tried it on Ultimate successfully, if you're curious.