This year was disasterous for me regarding machine troubles. I had to send my Acer laptop three times for repair (I have to say, their product is too fragile) and I also got Macbook white broken, which was worse, it was after 15 months since I bought it (out of warrant). And every time I got them broken, I suspend ongoing work and moved to some lightweight tasks. So that's how I started ongoing System.Xaml work these days.
Our System.Xaml.dll was initially written in last string, which was included in Mono 2.8 and had basic type system support. It also contained XAML readers and writers too, but they were far from usable. They were tested only in a few primitive use cases, and I didn't have much time to hack them either.
After System.Xaml.dll in our git master is a lot better. I have rewritten all of XamlObjectReader, XamlXmlWriter, XamlXmlReader and XamlObjectWriter with the actual object graphs. It also involved a lot of fixes in the type system (XamlType etc.).
It should work fine with ordinal constructs such as Type (TypeExtension), Array (ArrayExtension), List, Dictionary and IXmlSerializable (XData). But it still doesn't support lots of "settings", and attachable properties are not supported either (as we don't have WPF or anything that provides them).
Now that I feel I am done with basic features, I'd like to ask anyone who uses this System.Xaml.dll in .NET 4 try our System.Xaml.dll in git master and see if it works. Make sure that your Xaml usage is independent of things like WPF (we don't have it, so it won't work). To check this, there is MoMA.