Microsoft has made the November CTP of Avalon available to the general public. Previously it was only available to MSDN subscribers. Download the newest version here. It's exciting to see XAML and Avalon get a wider audience and Microsoft is actively encouraging public feedback to help shape future versions of the framework.
In order to avoid confusion, this version only works on Windows XP and Windows Server 2003, not Longhorn. More information on any installation issues can be found on the Avalon newsgroups.