Vista SP1 is what I did by the way. Twice.
Two different machines – two different Vista versions (Business on a HP 6715s and Ultimate on a Shuttle Small Form Factor SFF machine).
The world didn’t fall apart – it did seem like time was standing still for the best part of an hour but it completed successfully on both machines – no error messages – nothing to get worked up about.
I had hoped that the service pack might alleviate two problems I’d noted – one per machine. The first is a corruption of the Media Centre screen on the Shuttle and the other a hang or reset related to the Credentials Manager on the HP.
The credentials manager issue got so bad that I parallel installed Ubuntu 7.10 64BIT alternate CD to prove it wasn’t the hardware – see the article here
Well so far 1 of the problems seems lessened – the Credentials manager issue but the media centre problem remains.