From the ReadMe that comes with the WinFX SDK (CTP February 2006):
Xaml Runtime Parsing Exceptions:
We still have a way to go before the design/compilation process for xaml files gets smoother. In our first version, we aren’t going to be able to do as much content model and attribute value checking during compilation as we would like to. For hand coding xaml, until you start using a visual designer, it will be painful. To make it better, the XamlPresentation2006.xsd file has been significantly enhanced to give you better xaml editing in the xml editor. You can also choose to turn on Debug/Exceptions/Break when an exception is thrown for exceptions that you get when loading your xaml page. This will prevent .g.cs/.g.vb files from being opened which cause bad side effects. We realize that we need to invest significantly in this area in the future.
Now that is some good news and advise!