Microsoft has now rolled out the public release of Windows Vista SP1 RC. Follow the appropriate link:
Be patient, especially if you're going after one of the full install packages (which are over 500MB each).
When you install Windows Vista SP1 using the full install download, the system checks to see if updates discussed in KB937287 (updates servicing stack to permit the service pack to be successfully installed), KB938371 (updates installer and permits service pack to be uninstalled before the final SP1 is installed), and KB937287 (updates BitLocker support on Windows Vista Ultimate and Enterprise editions) have been installed. They've been released separately via Windows Update, but if you don't have them yet, they must be installed first. ( update 12-12-07 - Don't worry, the SP1 RC installer program will install them as needed).
If you use Windows Update to get Vista SP1 RC, you must install these updates before you will see Windows Vista SP1 offered as an update and they are displayed only after you install the update for Windows Update referenced earlier.
I'm downloading Vista SP1 RC right now, and I'll be back later with my first impressions. In the meantime, read the Windows Vista SP1 Guides for IT Professionals to get up to speed on the goodies awaiting you.