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' (unzip and run the cmd file) 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 we're all running our Vista machines here internally - we definitely RTM'd".

The RTM, or release to manufacturing, versions of both operating systems can easily be found on BitTorrent tracker sites such as the Pirate Bay, which has made them available since yesterday.  I am sure Microsoft will say, using anything except for files downloaded from Microsoft directly is a risk. So, if you plan to install SP1 via the registry changes/torrent, you are accepting this risk.