Windows Vista SP1 Release Candidate 1 is now available for download. If you are a TechNet or MSDN subscriber, the bits to get the process started are now on the subscriber download center. Those of you that are subscribers get to go to the head of the line and install right now. Everyone else will have to wait a few more days.
Want a subscription? See http://blogs.technet.com/keithcombs/archive/2007/12/06/technet-plus-direct-100-discount.aspx for more information.
[Note] I highly recommend doing a full backup with Complete PC, Ghost, TruImage or your backup software of choice before doing anything. You never know if something might happen and having a good backup is a best practice. You should also be aware that installing this RC1 turns on a time bomb that expires next summer. When SP1 ships in production form, installing it removes the time bomb.