It appears users are able to gain access to the RTM (or close to) version of the upcoming Update for Windows 8.1 by enabling a secret registry entry which will tell Windows to look in the correct places for the Update to be installed. It's currently unclear why this is possible, or if Microsoft is planning to patch this registry entry before launch in April.
So far, we've managed to install a huge 752MB update which has indeed bumped the operating systems build to 9600.17031. We've heard that this build is the final RTM, however these reports are not 100% certain so it's entirely possible that Microsoft could compile another build and call that the RTM instead.
We're downloading the update onto our main machine now. We're also checking our Windows RT device to see if the update appears on that architecture too. So far we've managed to install two updates, and are awaiting the actual Update 1 to appear. We'll keep you updated.
In the meantime, if you want to try this out yourself. Do the following: Press WINKEY+R and type regedit, and then navigate to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\", once there, right click on "Windows" and select new > Key. Name this new key "SHWindowsPoolS14" and then restart your system. Check for Windows Updates and install the two new updates which have appeared. Restart once those updates have installed and then check for updates again, you should see an even bigger update ready for you this time.
Remember, we don't advise you do this. We recommend you stay away from any registry tweaks and await the official launch in April.