Think about it: The GM is really for developers to finalize apps using new API's before submission to the app store. Developers must be on Macs as that is the only way to run xCode. Therefore, there is no official way to do it on a PC.
Using these 3rd party tools is risky and has resulted in problems for others.
I’ve never updated using an IPSW file with anything other than iTunes before, so it was a little nervy, but having just completed the update, it appears my data is intact.
Hopefully this proves to be true with a more detailed inspection.
I take it I won’t be able to backup on PC until iTunes is updated?