Grant Hinkson, a member of the Visual Design Team at Infragistics, has written a freely-available exporter that adds a custom panel directly within Fireworks that enables you to export your designs to XAML. You simply select the items you want to export, choose from a few simple options, then click a button to copy the XAML to the clipboard. From there, you can paste it into your tool of choice. Very slick and easy.
I also like the list of current features, their limitations, and what is planned for upcoming versions. Even better, you can send your feedback directly to their design team. Great job, guys!
While we're on the topic of exporters, I'd like to remind everyone of the permanent article I've planted on my site to collect all of these useful WPF Tools and Controls. If you're aware of any WPF-related tools or controls that I'm missing, please contact me directly. The page has become a very heavily trafficked resource, and it'll only grow as we get closer and closer to launch.