This is now available via Windows Update (it was only 66.9 MB on my machine via Windows Update).
Full file downloads:
Windows Vista Service Pack 1 Five Language Standalone (KB936330) 434.5 MB
Windows Vista Service Pack 1 Five Language Standalone for x64-based Systems (KB936330) 726.5 MB
If you don’t see it in Windows Update by days end, see:
Windows Vista Service Pack 1 is not available for installation from Windows Update and is not offered by Automatic Updates
Updated release notes:
Release Notes for Windows Vista Service Pack 1 (v3.2, 3/18/2008)
Also see the Windows Vista Team Blog post:
Windows Vista SP1 Released to Windows Update