This is a Simple mechanism for elemental arrows in vanilla minecraft snapshot 14w07a singleplayer
It basics read which arrow you have on Slot:0, and ativate a command for the arrow when you shoot.

Dragnoz did a similar project, but his tested fot the slot that you were selecting, my test for the arrow that you have on Slot:0 regardless of whether or not you are selecting it.

im not realy exited about it, because i havent been able to enable this mechanism for multiplayer, so I'll probably leave it the way it is