I just ran into this behavior with an attempt to pickle a dom tree for
an XML document whose nesting level never got deeper than nine child
nodes, and indeed it crashed the interpreter.  Throwing an exception
would be preferable, of course, to silent blowing up Python, but even
the exception seems to fly in the face of the documentation for the
pickle module [1] which claims (summarizing) that serializing recursive
objects using marshal will fail but pickling recursive objects will not
fail.  I can provide a repro for the XML/DOM pickling case if you think
that would be helpful, but that seems redundant since essentially you've
already illustrated the problem with your own repro case.  Thanks for
your work on the solution.

[1] http://docs.python.org/lib/node314.html