If I read this correctly, according to this Microsoft press release, they have signed off on the code for Vista and released it to manufacturing. This also means that you now have access to download it if you are a MSDN Premium subscriber. Please note: I don’t see the download as of yet, but this could change at anytime.

“The availability to MSDN® Premium subscribers of Windows Vista and the 2007 Microsoft Office system on release to manufacturing.”

Let us know if you have any questions and good luck with the testing and exploration.