After being chased off by a couple people who like to kill other players, I thought it would be nice if there were some way to opt out of PVP for those who wish to, while allowing it to continue for those who enjoy that aspect of the game. The idea I settled on I call a "vow of peace".
1) Player seeks out a NPC, who should be pretty close to a young player's start area (somewhere reachable by a new character without too much difficulty). They express their desire to take a vow of peace.
2) NPC checks that the player is not currently marked as a player-killer, and refuses to grant the vow if so (mark must be cleared first, which I believe is handled after two weeks of not attacking other players and after interaction with another NPC; the details here are inconsequential to this feature)
3) If the player is clean, they are bound to the vow. They can no longer be attacked by other players, however they also cannot attack other players.
3a) osl suggested an interesting twist, namely that players under a vow of peace are attackable, but the damage that would be done to them is instead done to the attacker. This would be an interesting twist, but may be harder to implement, and may detract from the players who do enjoy PVP. It would be a surprise to those who grief other players by attacking them randomly, however care would have to be taken to make sure people cannot game the system and use a vowed player to "train" themselves.
If a player later decides they wish to participate in PVP battles, they need only to see the NPC and express their desire to break the vow. It can be undone immediately, and can be retaken as long as the player is not marked as a player-killer (no other "cool down" required).
Thoughts welcome, come find me (Rocinante) in IRC.