While I'm not entirely sure of how Avalon will work, I imagine VG.NET is an excellent preview of what is to come. You can use it as a transitional technology until Avalon is complete or until Longhorn becomes mainstream.

It's a very powerful animated vector graphics library that can be used anywhere .NET is used today and even has an IDE snapin that allows you to draw graphics that can be translated right into C# classes (and maybe also VB.NET, but I'm not sure.) 

It also has MyXAML support.

You can find out more about that here:
http://www.vgdotnet.com