Chris makes a note of the Avalon Tech Preview being available for MSDN subscribers (note to subscribers: it's not called Avalon in MSDN but simply WinFX SDK under Platform Tools,SDKs)
There's a document here that talks about what's in and what's not in the Preview.
Now that Avalon is going to be fully supported on WinXP and 2003 servers - it will be interesting to see if Mike's right about Winforms.
One of his arguments was that WinForms run on Win98 boxes, etc. My big concern would be how fast? I've always experienced "lags" on DotNet apps even on my XP or 2003 boxes - I'm expecting much of the same from Avalon - so if they both are slow in loading, then would you really recommend such a solution for lower-end boxes such as 2000 and Win98?
My experience may not be typical and there are probably some crack applications that likely run circles in DotNet but many posts I've read leave me with that "sluggish" impression.
The future for interface design is in XAML and XUL and similar technologies - now we just have to get all of our hardware up to spec.