Microsoft has said that Windows Vista SP1 is released to manufacturing and it should become available on Windows Update and the download center mid March.
OEM partners should start getting SP1 through and you should soon see SP1 pre-installed.
The retail version has also gone to manufacturing, too.
Mid April is the schedule for those users that have chosen to update automatically.
According to Microsoft, this SP includes changes to improve performance and reliablity. For example, up to 50% improvement in performance copying and moving files on the PC and network.
In addition, Microsoft uncovered some problematic driver issues during the Beta stage of SP1 development. These drivers were incorrectly installed and, if you have one these drivers, Windows Update will not permit the automatic install. Users can override this and install the update if they wish. Apparently, the drivers simply need to be reinstalled.
I would imagine that these driver issues will become better known in due course.