The WPF and WF teams have worked together to make sure our markup formats are both XAML.
See a bit about using XAML for Workflow in this article. Comments on the article should go on this post from K. Scott Allen.
Only one (very minor) problem I saw in the article...Grid.Row and Grid.Column start at 0, not 1. WPF should have thrown an exception, I would think in that case...not sure why we don't. I'll check why we aren't throwing now...
What other declarative formats would work well being described in XAML? Would love to hear your thoughts/feelings on that...