There should be a /battle [player name] [wager]
For example, if Notch executed /battle Steve 25, it would send Steve a request saying that Notch has requested to PvP, and he can do /battleaccept to accept it. This would be useful for servers who only allow consented PvP, such as mine. I get a lot of reports of unconsented PvP, and this is a complete solution.