TogglePVP is a very simple bukkit plugin that provides commands to quickly change the PVP setting in a world. Access is controlled through the permission "togglepvp.command".
Commands:
/togglepvp [world] - toggles the PVP setting in a world. If world is not specified, the sender's current world will be used.
/setpvp [world] - sets the PVP state in a world. State can be enabled with "true", "on", or "pvp", anything else will disable PVP. World can be specified or not specified as with togglepvp.