Don't know how I missed this interesting bit of news, but Andy Boyd mentions that the Avalon November 2004 Community Technology Preview is now available via MSDN subscriber downloads (255.39MB ISO). You can also find the WinFX SDK documentation online. From the download description:
The Community Technology Preview Edition of the Microsoft® WinFX™ Software Development Kit (SDK) contains documentation, samples, command-line compilers, and tools designed to help you develop managed applications and libraries using WinFX, which is the set of next-generation managed APIs provided by Microsoft. This release includes the .NET Framework 2.0 and the set of presentation classes code-named “Avalon.”
The Microsoft WinFX SDK requires the Microsoft .NET Framework v2.0 Beta 1 and the “Avalon” Community Technology Preview. In order to use Visual Studio 2005 Beta 1 to develop “Avalon” applications you must install Visual Studio 2005 prior to installing the WinFX SDK.
Although I don't see it mentioned specifically, it's my understanding that this SDK works with Windows XP.