The leaked Vista update we reported about is now official. Microsoft calls this: "An update is available that improves the performance and reliability of Windows Vista".

This is not the Vista SP1, it is just an update. It is somewhat unusual to offer such an update. Microsoft says that the update can achieve better performance and responsiveness in various scenarios. It looks like Microsoft assumes that the issues are not affecting all users. If the update would be critical it would be distributed via the Vista update service or marketed as a service pack.
The first Microsoft Windows Vista update is available for download on this Microsoft page.