Solved: Silverlight Spy on x64 platforms
As of version 2.0.0.15 Silverlight Spy supports x64 platforms. To make this work I was required to rebuild the Spy with a modified target platform. The consequence of this is that all previous versions of the Spy are no longer able to check for updates. If you installed the Spy previously, you will see a 'failed to check for updates' warning in the output pane when the Spy starts.
To resolve this warning you need to uninstall the Silverlight Spy and re-install it by selecting the install link on the Spy download page. Apologies for the inconvenience.
More installation improvements are on the way; I am in the process of getting rid of the manual csexwb prerequisite setup by creating a registration-free COM library. If this works out, the Spy no longer depends on manual setup steps and all bits will be downloaded and installed automagically using ClickOnce.
Kudos to Jason Tadros for helping out on testing the various Spy builds on x64 and to everyone else offering to assist me in getting the spy up and running on x64 platforms.