I also agree with Mapleleaf. Alliances are alliances, after all.
If I start a game, and found that a given player is a person I can trust, it is likely I will trust him in the future, in new plays.
It is not "a predefined alliance" but rather "a bias towards people with known behaviour". Of course I can choose to stab them, but that will likely diminish my trustability in later games among them, and so I need more reasons for stabbing than being "ridiculous to do anything but that".
The problem here is that everybody insists in play single games as plain Diplomacy. But the fact that there is a point system and a ranking has to be taken into account. Using that, and the history of games, you can analyize the behaviour of your co-players, by looking at their stats, previous games, etc.
I usually study my co-players when entering a game.
@Kestas: perhaps what can be (easily?) done is some way to be able to query all the games where two persons have played together, and so be able to determine how likely is that they will ally again...
As a side effect, you will have a way to simplify the detection of multiaccounters.
A more complex thing may be some kind of wiki, where anyone (with certain level in the game?) can inform his discoveries, as comments attached to the players. (Something as "Aries is known to be usually allied with BlackFire and LynxPhoenix. They are between 1 and 3 Argentinian guys, and their alliances are almost unbreakable"). That would have helped me, Otto Von Bismark, and the rest in Downfall (gameID=5886). Luckily, some of us managed to discover their alliance in time, and dealt with it, as mapleleaf suggested.