A preliminary version of the Aero User Experience Guidelines is avaliable for download. There is a lot of content yet to be added, but what is in there looks pretty good. Here's an interesting snippet:

If an application’s design calls for additional glass surfaces, developers can utilize this visual treatment through the Aero Glass API. Using glass in your application is not something everyone should do. You don’t need to have glass in your UI to make a great Windows Vista application. Glass is best used on areas without dense information or interactive UI. The objective is to reduce the “weight” of the surface of the application to focus on the most important parts of the UI. Preferably use glass in surfaces touching the window frame, visually integrating your application with the window border for a more cohesive “look and feel”.