While the discussion on non-consensual PvP is important, it seems that the design of PvP will involve penalties (or at least consequences) to unprovoked aggression.
Now, whether it's client or server handled is a different thing, but the idea is the same: While allowed, there will be some sort of game-based deterrent to picking fights w/ people w/o their consent.
However, the amount of effectiveness that this deterrence might have is totally dependent on what it is, and how it's enforced. Anyone have any cool ideas that people would be satisfied with.
On a side note, are player corpses lootable?