Hello everybody:
In my experience with MMORPGS (and they are quite a few) the only PVP systems that were enjoyable and viable were faction based and limited to certain areas.
First of all the factions (which can be anything: Deity, Race, Good vs Evil, Realm, etc...) give the PVPing experience a role-playing foundation. Besides it filters out the possibilities to have wild PVP-players which would spoil the playing experience. It will also increase the teamwork feeling and grow certain bonds between players.
In order to make such a system viable it would have to be limited to certain areas where peole of separate factions can confront each other. For instance the Cities and their immediate surroundings will be safe due to a high concentration of guards which protect the citizens of that city, however the farther you get into the wilderness the more chances you have to get attacked.
This will encourage players to group up if they want to go hunting on certain monsters which will be only in those areas.
It is just a tought, but in my opinion it is the most viable PVP system. Especially since in Planeshift there are no levels it would allow less experienced players to enjoy the relative safety of the city and it\'s surrounding areas before adventuring into the wild.
Off course the dead realm will be an entirely non PVP-zone to avoid endless dead loops. =p