I read this piece on the Zdnet web site concerning activation for vista and how it may differ from activation in the past. It gives advise on how many times you can do it for OEM and retail copies. Plus advise if your an 'hardcore re-installer'!
Microsoft releases details on Vista activation | Ed Bott’s Microsoft Report | ZDNet.com
Microsoft releases details on Vista activation | Ed Bott’s Microsoft Report | ZDNet.com