Player-versus-player actions in tabletop RPGs are a tricky subject. Matters recently came to a head in ACKS, when people got fed up with the actions of a particular chaotic PC mage against their holdings (secretly charming their henchmen and such). A long and heated discussion ensued, and I think we reached a satisfying conclusion.
It came up that we had managed to have a lot of fun with PC vs Group actions during my Traveller game some semesters ago, and an analysis of those actions followed. Two actions in particular stood out. In the first, which I've mentioned before, Tim's character was tasked with jettisoning some prisoners into the sun while the rest of the party was away buying spare parts. Instead, he made a deal with some other shippers and managed to send the prisoners home. The rest of the players know about it, and thought he had erred badly in permitting their enemies to live, but their PCs knew nothing of it, and it worked out for the better.
In the second, Jared's character pulled one over on Tim's character in order to get the party to pursue his interests. I don't remember exactly how it went down, but they both knew exactly what was going on, and Tim was OK with it. And it was from this case that we derived a general principle for Acceptable PvP - the target needs to agree to it for whatever reason. Maybe they think it will advance a story in a pleasing fashion, maybe they want to get rid of a particularly troublesome henchman, or what-have-you. But in the end, the affected player needs to not have a problem with what is happening. This says nothing about the characters, being an entirely metagame concern, but does serve to mediate out-of-game issues.
This marks an interesting shift in the ACKS campaign. The players have abandoned their freewheeling, backstabbing roots in for solidarity and common goals. I believe something to the effect of "Look, let's delay fighting among ourselves until it's safe to walk around out in the wilderness" was actually said. While I will miss the old propensity towards PvP (which I always found pretty entertaining from my side of the screen, especially if undertaken in a dangerous environment like the dungeon), if the players are happy about it then it is probably better for the campaign. I also expect we may see the Acceptable PvP Principle propagate into other games we play here.