kiaerulf, when backstabbing is implemented no doubt it will be done in a way that won't compromise the fun of both parties. I highly doubt you'll be able to stab anyone you want, for that is Player Killing and we do not tolerate such griefing, nor have plans for it. We're not going to touch that skill in a very long time, so everything is a guess at this point - however, it's possible it will only be allowed in designated PvP areas, or you'll have to mark your character in a sort of "auto-accept."
The problem with roleplaying this purely using text instead of supporting game mechanics, is it becomes a battle of words, and players aren't always willing to compromise. When we put features into the game, we try to make sure they are as fair as possible and leave little in the hands of the players to disagree on.