As a result of overwhelming customer feedback, we've now made the Avalon November CTP available to anyone who wants to get their hands on it, whether or not you've got an MSDN subscription. This is the first release of Avalon that supports Windows XP and Windows Server 2003. It's pretty stable and functional, as far as CTP releases go, and it's a great place to start if you want to experiment with what will become the strategic presentation technology for Windows over the next decade. For more information on what's contained in the CTP release and how to go about evaluating it, check out this MSDN article.
You can use any Beta 1 release of Visual Studio 2005 to code for the Avalon CTP (including the Beta 1 Refresh); check out this article on using the Express products with Avalon. Be aware, however, that later CTP releases of Visual Studio 2005 aren't compatible with this Avalon CTP drop - that's the price you pay for being on the bleeding edge! The Avalon CTP release ships with an SDK containing a fair quantity of documentation; this is also available online here. Lastly, some other useful blogs to read on Avalon include those from Chris Sells, Daniel Lehenbauer, Chris Anderson, Nathan Dunlap and Karsten Januszewski.