Several technologies have leveraged XAML since .Net 3.0 : WPF, WF, Silverlight, XPS.
State of XAML 2006
In .Net 4, we will ship XAML 2009.
XAMLPad enhancements to represent the XAML Data Model is too flexible : OMV (Object – Members – Values
System.Xaml.dll in .Net 4.0.
in .Net 4.0, WF vocabulary will get richer (DataFlow) and a rich WCF vocabulary will be introduced. Thus, you can have an entire application (UI + Business Logic + Services invocation + Workflow) implemented in XAML.
To benefit from XAML, have your framework XAML ready. Ride the XAML wave.