My colleague Pete has it up on his site: http://www.blois.us/Snoop/
This developed out of internal tools originally built into Expression that help us inspect our visual tree. Originally we dumped the tree to a text file, then Pete put some UI on it, then Kenny incorporated the UI into Expression, and since then Pete has raised the stakes repeatedly making it standalone and adding features such as being able to inspect and even set values in the tree, and to be able to set breakpoints on data changes.
Yes, you can debug databinding in WPF using Snoop. Check out the link for more.
Highly, highly recommended!