i tried it with win7 and have the same issue.
i have filed a bug with lots of details and logs:
cannot join PDC domain with windows 7
if you could also turn debug on on and make one attempt to join (**after adding the root user and password to samba,
PER THE DIRECTIONS**), capturing the logs cleanly (i *think* i have, but not 100% sure), it would be of most help.
it took it a while to recognize things, so you may want to wait a minute or so to attempt things.
there is a mention of active directory in the error message. we're not supposed to be using active directory.