Reviewing the current buildbot failures, this is still the only one reporting the error:

http://buildbot.python.org/all/builders/AMD64%20Ubuntu%20LTS%203.x/builds/3165/steps/test/logs/stdio

I can reproduce the failure locally by running:

$ PYTHONWARNINGS=d ./python -m test test_venv
[1/1] test_venv
/home/ncoghlan/devel/py3k/Lib/imp.py:32: PendingDeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  PendingDeprecationWarning)
/tmp/tmpwmsqtb9y/pip-1.5.rc1-py2.py3-none-any.whl/pip/wheel.py:127: DeprecationWarning: This method will be removed in future versions.  Use 'parser.read_file()' instead.
test test_venv failed -- Traceback (most recent call last):
  File "/home/ncoghlan/devel/py3k/Lib/test/test_venv.py", line 289, in test_with_pip
    self.assertEqual(err, b"")
AssertionError: b"/home/ncoghlan/devel/py3k/Lib/imp.py:32:[160 chars]g)\n" != b''

1 test failed:
    test_venv

I'm updating the test to make sure that venv runs the subprocess in isolated mode.