Some answers to this are on the bug ticket rt.cpan.org/Ticket/Display.html?id=65735, and have come from a combination of Daniel Macks and mcmahon. For one thing, threads on Windows have been supplanted by ithreads after Perl 5.10. In addition, a nice patch by Gaspar Chilingarov seems to have made a lot of headway today towards solving the problem.
The t/thread.t tests are simply skipped if version >= 5.10. The only remaining problem is in telling the test harness/TAP - that it is okay not to have any tests for this. Just printing out "ok" to STDOUT or STDERR before exiting does not work. I will break out my old book - Perl Testing .. A Developer's Notebook - and figure this out.
I should be able to get it from here, but if anyone could spare me the reading, that would be great.