In case you haven't seen other pointers to Chuck's blog, please make sure to visit him regularly. Chuck describes his role in the development of XAML in his first of several posts describing XAML.
Thanks to Chuck's influence and focus, XAML is evolving into a language that is generically useful. WPF's usage of XAML is huge, but we'll see other teams (like Windows Workflow Foundation today) begin to use it as well.
Chuck spends most of his time currently working "Cider", the designer for WPF that will ship with Visual Studio "Orcas". He has several posts pointing to msdn, channel 9 and pdc videos of Cider.
I've learned a lot from Chuck. Whenever I meet with him I usually realize I still have much more to learn from him...