If you are one of the few who don't have Longhorn or one of the others who have it and decided it may not be worth the trouble at the moment or if you just want to play with XAML's declaritive programming model today on a stable platform that works, check out XAMLON.
I know XAMLON isn't new, but the newest version, 0.7 (beta 3) has just been released.
“This version adds support for MDI windows and Menus in your markup. Two new examples have been added to show examples on how to use these new features.
Additionally, 'Xamlon.exe' has been added which allows for the execution of .xaml files without entering a license key.”
This Beta expires in May, but the license key is free (for now). This gives you a way to toy with XAML without the trouble of running Longhorn. You don't get all of Longhorn's features, of course, and you don't get the same version of XAML that will be in the final release a couple years from now, but you can say the same thing for about the Longhorn PDC bits, right?