Vista SP1 to make an appearance tomorrow
Do note that unlike the Automatic Update version scheduled for release on or after April 18, the End-User Manual Update is optional. It will not automatically download or update your Windows Vista installation even if you have already chosen to automatically receive Windows Updates. You have to head over to the Microsoft Download Center or Windows Update site and manually select the optional Service Pack 1 upgrade.
This will change on or after April 18, when the Automatic Update version is released. That release will automatically download or update your Windows Vista installation, if you have chosen to receive Windows Updates automatically. If you do not wish to automatically install Service Pack 1 for Windows Vista, be sure to disable the automatic installation of updates.
So you’re going to have to manually check the Windows Update site unless you can hold off another month. For reference, SP1 was originally slated for launch February 19th, though Vista’s driver woes delayed the update.