Having further investigated, I'm convinced that this is a bug.

Attached are two minimal C programs.  Both create threads that do
nothing but sleep for 2 seconds and then exit.  The parent tries to join
with the child thread, with a timeout of 10 seconds.

The only difference is that test1 uses scm_spawn_thread, and test2 uses
scm_call_with_new_thread.  test1 always times out, and test2 works
properly.

Having looked at the code in threads.c, I'm surprised that either of
them work.  Both scm_spawn_thread and scm_call_with_new_thread arrange
for pthread_detach to be called in the child (in spawn_thread and
launch_thread, respectively).  This is supposed to make the child
un-joinable.

I've attached not only test1.c and test2.c, but also the diff
between them.  I compiled them as follows:

  gcc -o test1 test1.c $(pkg-config --cflags --libs guile-2.0)
  gcc -o test2 test2.c $(pkg-config --cflags --libs guile-2.0)

Any ideas?

   Thanks,
     Mark