It looks like Microsoft have pushed the button and released SP1 via Windows Update. It's not being pushed down via automatically yet, that is coming mid April but it should be listed on Windows Update for download now.

If you want to know all about Windows Vista Service Pack 1 listen to this weeks The Media Center Show with Ed Bott, Ed will be explaining all about the update

If you don't see it checkout the list:

Cause 1
You are already running Windows Vista SP1.
Cause 2
Windows Vista SP1 has not been released for the language of the language pack that you have installed.
Cause 3
Windows Service Pack Blocker Tool is used to block the delivery of Windows Vista SP1 from Automatic Updates or from Windows Update.
Cause 4
You tried to install Windows Vista SP1, and the installation failed with a known inconsistency in the file or registry structure.
Cause 5
A hardware device driver or device software was problematic when you updated to Windows Vista SP1.
The Windows Update service can detect the small set of device drivers and software that falls into this category. The Windows Update service will not offer Windows Vista SP1 until an update for the hardware device driver or the device software has been installed.
Cause 6
You have installed a prerelease version of Windows Vista SP1, and you must uninstall the prerelease version, or start with a new installation of Windows Vista.

Windows Vista Team Blog : Windows Vista SP1 Released to Windows Update