You can finally download the Vista Beta 2 bits by enrolling in the Customer Preview Program. Note that the WinFX Runtime Components (needed to run WinFX apps) are automatically installed in this Windows Vista build. Therefore, if you're looking to just run WinFX applications on Windows Vista, installing Vista is all you need to do.
I would strongly suggest using the Get the Beta page to determine what to download and in what order as it covers the different scenarios - such as "I want to develop WinFX apps" , "I want to develop only native (non .NET) apps", etc.
Also note that the links on the Get the Beta page are to the bits that are in alignment with Vista Beta 2.