You're right, the python3 executable is indeed

20:05 ~/tmp $ python3 -V
Python 3.2rc1+

The relevant .py files (mailbox.py, email/*.py, (email/mime/*.py), test/test_mailbox.py are symlinked to the repo files.
(And, different to this morning - sorry, *sorry*: lunch break - they all point to HG:33774ca03c96 (SVN:r88332) files.
I've removed my patch and this is what happens (only five errors remain).
The traceback:

Traceback (most recent call last):
  File "test_mailbox.py", line 2092, in <module>
    test_main()
  File "test_mailbox.py", line 2087, in test_main
    support.run_unittest(*tests)
  File "/Users/steffen/usr/lib/python3.2/test/support.py", line 1145, in run_unittest
    _run_suite(suite)
  File "/Users/steffen/usr/lib/python3.2/test/support.py", line 1128, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: multiple errors occurred

==========
This is the stripped output:

test_add_binary_file (__main__.TestMaildir) ... ERROR
test_add_binary_file (__main__.TestMbox) ... ERROR
test_add_binary_file (__main__.TestMMDF) ... ERROR
test_add_binary_file (__main__.TestMH) ... ERROR
test_add_binary_file (__main__.TestBabyl) ... ERROR

======================================================================
ERROR: test_add_binary_file (__main__.TestMaildir)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_mailbox.py", line 146, in test_add_binary_file
    sys.exit(1)
SystemExit: 1

======================================================================
ERROR: test_add_binary_file (__main__.TestMbox)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_mailbox.py", line 146, in test_add_binary_file
    sys.exit(1)
SystemExit: 1

======================================================================
ERROR: test_add_binary_file (__main__.TestMMDF)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_mailbox.py", line 146, in test_add_binary_file
    sys.exit(1)
SystemExit: 1

======================================================================
ERROR: test_add_binary_file (__main__.TestMH)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_mailbox.py", line 146, in test_add_binary_file
    sys.exit(1)
SystemExit: 1

======================================================================
ERROR: test_add_binary_file (__main__.TestBabyl)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_mailbox.py", line 146, in test_add_binary_file
    sys.exit(1)
SystemExit: 1

----------------------------------------------------------------------
Ran 327 tests in 8.540s