Revision 170 has been just committed. With this revision, test suite passes with Python 3.2, but still fails with 3.1:

$ python3.1 -m testtools.run subunit.test_suite
Tests running...
======================================================================
ERROR: subunit.tests.test_subunit_filter.TestFilterCommand.test_default
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/subunit/python/subunit/tests/test_subunit_filter.py", line 323, in test_default
    "test: foo\n"
  File "/tmp/subunit/python/subunit/tests/test_subunit_filter.py", line 312, in run_command
    raise RuntimeError("%s failed: %s" % (command, err))
RuntimeError: ['/usr/bin/python3.1', '/tmp/subunit/filters/subunit-filter'] failed: b'Traceback (most recent call last):\n  File "/tmp/subunit/filters/subunit-filter", line 158, in <module>\n    main()\n  File "/tmp/subunit/filters/subunit-filter", line 153, in main\n    forward=False)\n  File "/tmp/subunit/python/subunit/filters.py", line 92, in filter_by_result\n    result = result_factory(output_to)\n  File "/tmp/subunit/filters/subunit-filter", line 150, in <lambda>\n    lambda output_to: _make_result(sys.stdout, options, filter_predicate),\n  File "/tmp/subunit/filters/subunit-filter", line 130, in _make_result\n    TestProtocolClient(output),\n  File "/tmp/subunit/python/subunit/__init__.py", line 614, in __init__\n    stream = _make_stream_binary(stream)\n  File "/tmp/subunit/python/subunit/__init__.py", line 1267, in _make_stream_binary\n    return _unwrap_text(stream)\n  File "/tmp/subunit/python/subunit/__init__.py", line 1281, in _unwrap_text\n    if type(stream.read(0)) is str:\nIOError: not readable\n'
======================================================================
ERROR: subunit.tests.test_subunit_filter.TestFilterCommand.test_tags
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/subunit/python/subunit/tests/test_subunit_filter.py", line 336, in test_tags
    "tags: a\n"
  File "/tmp/subunit/python/subunit/tests/test_subunit_filter.py", line 312, in run_command
    raise RuntimeError("%s failed: %s" % (command, err))
RuntimeError: ['/usr/bin/python3.1', '/tmp/subunit/filters/subunit-filter', '-s', '--with-tag', 'a'] failed: b'Traceback (most recent call last):\n  File "/tmp/subunit/filters/subunit-filter", line 158, in <module>\n    main()\n  File "/tmp/subunit/filters/subunit-filter", line 153, in main\n    forward=False)\n  File "/tmp/subunit/python/subunit/filters.py", line 92, in filter_by_result\n    result = result_factory(output_to)\n  File "/tmp/subunit/filters/subunit-filter", line 150, in <lambda>\n    lambda output_to: _make_result(sys.stdout, options, filter_predicate),\n  File "/tmp/subunit/filters/subunit-filter", line 130, in _make_result\n    TestProtocolClient(output),\n  File "/tmp/subunit/python/subunit/__init__.py", line 614, in __init__\n    stream = _make_stream_binary(stream)\n  File "/tmp/subunit/python/subunit/__init__.py", line 1267, in _make_stream_binary\n    return _unwrap_text(stream)\n  File "/tmp/subunit/python/subunit/__init__.py", line 1281, in _unwrap_text\n    if type(stream.read(0)) is str:\nIOError: not readable\n'

Ran 256 tests in 0.867s
FAILED (failures=2)