To be honest, I wasn't really happy either with having to return HOST, 
it's somewhat redundant given that all these tests should be binding 
against localhost.  What about something like this for bind_port():

def bind_port(sock, host=''):
    """Bind the socket to a free port and return the port number.
    Relies on ephemeral ports in order to ensure we are using an
    unbound port.  This is important as many tests may be running
    simultaneously, especially in a buildbot environment."""

    # Use a temporary socket object to ensure we're not 
    # affected by any socket options that have already 
    # been set on the 'sock' object we're passed. 
    tempsock = socket.socket(sock.family, sock.type)
    tempsock.bind((host, 0))
    port = tempsock.getsockname()[1]
    tempsock.close()
    del tempsock

    sock.bind((host, port))
    return port

The tests would then look something like:

HOST = 'localhost'
PORT = None

class Foo(TestCase):
    def setUp(self):
        sock = socket.socket()
        global PORT
        PORT = test_support.bind_port(sock, HOST)

So, the return value is the port bound to, no change there, but we're 
abolishing preferred_port as an optional argument, which is important, 
IMO, as none of these tests should be stipulating which port they want 
to listen on.  That's actually the root of this entire problem.