Fiddler has many useful extensions. However, I can't find one that understands WCF's binary-encoded SOAP envelopes. (Content-Type: application/soap+msbin1)
Suggestions for comparable (non-Fiddler) tools are welcome too.
After lots of work on ancillary stuff that doesn't really matter (eg decided to roll my own quick-n-dirty recursive descent XML parser), I've just committed the first checkin where the plugin actually does its job in a usable fashion. Still extremely rough around the edges, with many features not implemented, but if you want to see binary WCF in Fiddler now's your chance!
Source code only for now: http://tfstoys.codeplex.com/sourcecontrol/changeset/view/26191?projectName=tfstoys#BinaryXMLInspector
I'll post binaries on CodePlex when it's polished enough for general consumption. Probably won't remember to update this "answer" though. If you're reading this months/years from now, be sure to click over to the Downloads page -- or at minimum, grab the latest source instead of this changeset.