XAMLON: XAML before Longhorn

I am amazed by all the early activity going on around Longhorn in the blogs I read, all the while Whidbey's release is still at least a year from now...
Anyways, waiting until Longhorn to benefit from declarative UI development seems like a long time. XAMLON is a library that attempts to bridge the gap by bringing a subset of XAML into the 1.1 .NET world.
Its approach is a bit different from Avalon's, as the XAML isn't compiled, but instead interpreted at runtime, and as a consequence the markup can't contain logic code. My guess is that's because the 1.1 framework doesn't support partial classes.

There are some other attempts at describing .NET UI with XML, like UISerializer and XML Forms.

In the non-.NET world, there is Glade XML (for GTK), XUL (used in Mozilla and some Java projects like Thinlets or Luxor XUL) and Swing XML (SwiXml).

Update: Andrew linked to another XAML rendering engine.
Miguel on XAML and other PDC topics.
XUL# and XBL#.

Update:
Beyond HTML: A Look at Markup Languages For Creating Rich "Classic Desktop-Style" UIs.

And now there's also www.myxaml.com, and open source initiative.