Python C extension modules can release the GIL while they're running which allows for true concurrency.  See this Google code search <a href="http://www.google.com/codesearch?q=python.org+lang%3Ac+Py_BEGIN_ALLOW_THREADS&#38;hl=en&#38;btnG=Search+Code" rel="nofollow">http://www.google.com/codesearch?q=python.org+lang%3Ac+Py_BE...</a> for example usage.