Adrian Kingsley-Hughes of zdnet has posted the information he got from a Microsoft spokesperson about it :
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.