Was just scoping out the .NET 4.0 poster here when I zoomed in on core and saw…
Looks like there’s movement on making XAML more available to developers for serializing plain old CLR objects. Unfortunately, my Google-fu is failing me; I can’t find any more information about what’s happening here…

You’ll have all kinds of custom serialization control in .Net 4. They announced a lot of good things at PDC this year.
In particular, you TL36: Declarative Programming Using Xaml had the info you want:
Presentation on Channel 9.
http://channel9.msdn.com/pdc2008/TL36/
Hope that helps.