Yesterday some people got a surprise - Windows Vista SP1 coming down the Windows Update pipes. Turns out someone at Microsoft flipped the wrong switch.
Here the information that I have thanks to a Microsoft spokesperson:
We’re very pleased with the progress we’ve made with Windows Vista to date. Since releasing SP1 to manufacturing on February 4th, we have made Windows Vista SP1 available to beta testers, MSDN and TechNet Plus subscribers, as well as Volume Licensing customers. Today, a build of SP1 was posted to Windows Update and it was inadvertently made available to a broad group. The build was intended only for our more technically advanced testers, and was meant to only be offered to those with a specific registry key set on their PC. For general availability, we are still planning to make SP1 broadly available in the mid-March timeframe.
So, I'm assuming that if you got SP1 yesterday, you were lucky, but if you're still waiting, you'll be waiting until mid-March.