If you insist on doing PvP, restrict where it can be done like on LOTRO for example where there is an area set out where you know pvp is taking place. That leaves the rest of the universe to the people who want to enjoy the storylines and great (i hope) sites.
If you do go pure Pvp, just remember this, every PvP game out there has been hacked to allow people to cheat, and also you don't exactly entice new players when they are likely to be ganged up against before they have set foot out the door. Also the fact that PvP will do everything they can just to ruin some poor by standers day. Gate camping, ganking, the works. And just think of this. As much as this is a game, just think about all the new people that will no even bother taking out a sub or whatever simply because of pvp'er who want to get their way.
You want this to be a success in terms of gameplay and business. Keep the world purely PvFO and have seperate areas specifically for PvP. Thatway, everyone is happy.
I am really looking forward to Fallout online but don't spoil it before it has even come for a bulk of people who want to really enjoy the universe.