Yesterday, many sources including ourselves reported that the 64-bit version of Vista SP1 was being distributed through Windows Update. And in fact, it was, and many people have already installed it because of this. As it turns out though, the update availability was a glitch as Microsoft did not intend to push the update out of the door just yet, but was only meant for their 'technically advanced testers', namely Technet subscribers and the like.
All glitches aside, Microsoft put the finishing touches on SP1 a while ago, but they are staging the roll out for various reasons such as certain known incompatibilities that could lead to further complications. This hasn't stopped people from distributing it anyway, despite of reports that indicate that the update does not bring any hard feature additions, and bug fixes are hardly noticeable unless you were having trouble with one of the few critical well documented problems on the OS.