Microsoft was vague this week about when Windows Vista SP1 RC1 would be available, but based on the other two releases yesterday, it was going to be soon, and here it is. You use Windows Update to download it, but first you have to download a tool from the site above to "mark" your machine as eligible for SP1.
Windows Vista Service Pack 1 RC is available through Windows Update. Experience the WU based Vista SP1 installation by following 4 simple steps.
I'm assuming that when SP1 is released they'll be a standalone installer you can download to avoid all this annoying install, reboot, check again, install, etc. The 4 simple steps Microsoft points out aren't really that simple. Well, maybe they're simple, but they are tedious. The annoying details are at the linked Microsoft site above.