There's a thread in the MVP list that discusses how to install Service Pack 1 for Windows Vista.
Quick summary - You add some registry keys, download the KB935509 hotfix and reboot. Then download KB937287 followed by KB938371 and your system should say "Windows Vista Service Pack 1".
One of my fellow MVPs who is part of the official Windows Vista Service Pack testing group writes:
It's not a Hack. Actually somebody has leaked the script which was provided on the Windows Connect site to get SP1 Beta available to Beta testers through Windows Update. That script was necessary to get SP1 if one wants it via WU path and was provided with the WU Release message document.