exarkun@boson:~$ cat > test_foo.py
from unittest import TestCase

class SomeTests(TestCase):
    def test_foo(self):
        pass
exarkun@boson:~$ python -m unittest test_foo

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
exarkun@boson:~$ python -m unittest test_foo.SomeTests
Traceback (most recent call last):
  File "/usr/lib/python2.5/runpy.py", line 95, in run_module
    filename, loader, alter_sys)
  File "/usr/lib/python2.5/runpy.py", line 52, in _run_module_code
    mod_name, mod_fname, mod_loader)
  File "/usr/lib/python2.5/runpy.py", line 32, in _run_code
    exec code in run_globals
  File "/usr/lib/python2.5/unittest.py", line 816, in <module>
    main(module=None)
  File "/usr/lib/python2.5/unittest.py", line 767, in __init__
    self.parseArgs(argv)
  File "/usr/lib/python2.5/unittest.py", line 794, in parseArgs
    self.createTests()
  File "/usr/lib/python2.5/unittest.py", line 800, in createTests
    self.module)
  File "/usr/lib/python2.5/unittest.py", line 565, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/usr/lib/python2.5/unittest.py", line 553, in loadTestsFromName
    test = obj()
  File "/usr/lib/python2.5/unittest.py", line 209, in __init__
    (self.__class__, methodName)
ValueError: no such test method in <class 'test_foo.SomeTests'>: runTest
exarkun@boson:~$ python -m unittest test_foo.SomeTests.test_foo
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK
exarkun@boson:~$ 

I don't really know what's happening in the middle case.  The first case
seems to fail because the test loader goes around looking for subclasses
of __main__.TestCase.  It only finds subclasses of unittest.TestCase,
though, which it rejects.