I think that the only time this becomes a problem is when an alliance is formed through a means of communication which is not universally accessible. If two players have played face-to-face in the past, they will still be able to play among a group of strangers, be it face-to-face, postal or php, provided that they don't communicate in ways that the other players cannot. To me, barring some telepathic link, this means that as long as people confine themselves to using this site as their sole method of communication, and don't use things like IM or cellphones, it should be possible to have a fair game with a group that contains both strangers and prior acquaintances.
That said, I would deem it unethical (and yes, one can lie about where his army in Galicia is going and still be ethical) for me to play in a mixed group of face-to-face friends and php strangers, since I would be unable to divorce my past experience with a player from my judgments of what he might do in the present game. This ethical dilemma does not exist within our little php world, since any player can draw conclusions based on the public information about an opponent's points and prior games.
So, to sum it up, I think that people who have played together outside of this format may ethically use php, but only in private games. If people are playing public games here, then they should not play with people they have played in other Diplomacy settings without first disclosing this information to the other players.
I can think of no reasonable way to enforce this, just as, in general, there is no reasonable way to enforce any ethical code. Except for guilt. But I don't think Diplomacy players have guilt.