We've released the Avalon and Indigo CTP for March, 2005 (otherwise known as the WinFX™ SDK) for MSDN Subscribers. The download includes documentation, samples, command-line compilers, and various tools to help you develop applications using Avalon. The download is a 444.67MB ISO CD image that can be found under Tools, SDKs, and DDKs | Platform Tools, SDKs, DDKs | WinFX SDK - Community Technology Preview. We've also posted the release notes and updated WinFX SDK documentation.
According to Brad Abrams, these bits work just fine with the latest Visual Studio 2005 CTP. I haven't spent as much time with Indigo, but Avalon is very exciting! If you have the time, please download this release and provide us with your feedback.
Update: Our own Karsten Januszewski has provided a very helpful article about what's new for Avalon in the March 2005 CTP.