Windows Vista UX Guidelines (preliminary) download
preliminary draft of the Microsoft Windows Vista User Experience
Guidelines (or “UX Guide” for short). These guidelines contain
information on What’s New in Windows Vista, design principles,
guidelines for controls, text, windows, and aesthetics. They will help
designers and developers create high quality, consistent Windows Vista
Update 13 Sept 05: Chris Pririllo provides a good summary.
"Looks like the AERO User
Experience has a blog, too - although nothing's there yet."