@Thucydides
I won't go into the benefits of being stabbed, but I don't seem to get your point. I rephrased two of your comments into 1 sentence, so they're better comparable.
Now you say: "distrusting a person, just because you've met him/her before and (s)he did stab you (on multiple occasions) is allowed."
However, you stated earlier: "trusting people, just because you've met them before and he didn't stab you -and you somehow start to like the guy/girl- is not allowed."
To summerize your two comments: you're only allowed to cooperate with people you don't trust. Because, as soon as you trust the guy --or girl but somehow I expect the ratio not to be 50/50, thus guy will be more occuring-- and somehow might get to know him/her, your behavior is considered meta-gaming. Of course, I'm not so naive to think that every alliance will end up in a friendship of some sort. But my point is this: I'm not planning to prevent becoming a friend on this site from happening, ergo this could happen. If it happens, you're labeling me as being a meta-gamer (I couldn't find the rules, but I guess it's as severe as multi-accounting), even if this friend and me are cooperating because we both see benefits in an alliance.
@Chockney: with this many games going on, I doubt that mods are able to keep track of everything that's said and that this merely depends on whether or not fellow players consider the on-board events as meta-gaming, which might also just be a 'normal' stab or a 'normal' change in alliances.
@Draugnar: I couldn't help it, but I agree again: that what might be percieved as lying, stabbing, telling the truth, meta-gaming, etc.. depends to a large extent what your (on-board) advantage is. I think it's highly dubious to label some-one as a meta-gamer, and I honestly can't see the difference between a cocktail of (1) a bit of luck in guessing the right defense, (2) good negotiations and (3) a well-payed swindle; and the concept described here as "meta-gaming".
Then I have several related questions: What if I know a player missed the last two phases in game A (1-day deadline), am I allowed to make a move against him in game B (25-hour deadline)? What if I'm in a full attack against him? What if I though (beforehand) this was the right time for a stab? What if my alliance partner asks my opinion about a plan of attack?
I would consider them all meta-gaming, even if you would not use the information acquired from any outside-of-board events, but I somehow sense not everybody will agree with me on this.
BTW, in real-life diplomacy spying/trying to overhear others/stealing orders -as long as they're not in their final place- is allowed, is there any chance this might be implemented in online-diplomacy? :)