It looks like Microsoft has another prerequisite for installing Vista SP1 on a laptop. You have to have it plugged in to the AC adapter, or you cannot continue with the installation. At the moment, I had 50% battery life remaining, which was about 2 hours, and I attempted to install SP1. The message was: "Setup requirements" "Before installing Service Pack 1, plug in your mobile PC power cord to ensure that your computer maintains power during the installation." (See attachment for screenshot) I was denied until I plugged in the AC adapter. I guess it's kind of a smart idea. Running out of battery in the middle of a Service Pack install would NOT be a good thing!
Yes, I now noticed it mentioned in the deployment guide for SP1. The deployment guide isn't going to be something the average joe reads though. And, even at that, the deployment guide has it listed as a "predeployment task", along with "Back up data", "Read the release notes"... etc. It doesn't state that it's a requirement.