The Avalon November 2004 Community Technical Preview for Windows XP has been released on MSDN Subscriptions as part of the WinFX SDK.
"Avalon" requires a Beta 1 build of .NET Framework 2.0 to be installed.
Update: Chris Anderson has a demo application -- XamlPad. Note that he is using ClickOnce to deploy updates (this is the first time I've seen that technology working).
Gaston Milano (the WhidbeyCommands for Visual Studio 2003 developer) has released a Xaml Viewer for Visual Studio 2005 which has been updated recently.