By now many of you may have seen reports of Microsoft Windows Vista Service Pack 1 in the wild. For a while, it was merely speculation (and several good hints) towards Release Candidate Refresh 2 (RCR2) being the Release to Manufacturer (RTM) build. There has been a "registry hack" going around for a few days now to enable the option to download SP1 via Windows Update. This "hack" is actually straight from Microsoft, which was given to beta testers on Connect January 23rd.
According to Geeks Anatomy, Jeff DaVos (a Windows Vista SP1 beta program leader) has said on the Connect news group that this is indeed the same release being used for RTM, and that "What you got is what were all running our Vista machines here internally - we definitely RTMd".
Link: Geeks Anatomy