Scary moment!
I flashed my BIOS (down from F9 to F7) and upped the overclock on my PC tonight. On rebooting the PC, Vista picked up new hardware (Some Intel PCI communication bridge and Texas Instruments IEEE1394 host controller) - presumably the BIOS flash changed the hardware signature.
As a result Vista (OEM 64bit premium purchased from Scan) said that it needed to be reactivated. I tried the electronic route and it said that key had already been used and I could not use it again
I had to go through phone activation and state that my Vista was only on one machine and then quote 48 digits to the operator to get a new key...
So a word of warning to those of you who plan to flash your BIOS often - you may need to go through activation again...