I realize talents will change quite a bit with Cataclysm but assuming Silence stays where it has always been, PvP disc priests will have the option of going down Shadow to get Silence as well as getting a mortal strike effect with Mind Blast (supposedly all mortal strike effects between classes will be equal). Disc priests are already the most offensive healer in arenas, with silence and mortal strike a disc/shadow build could be unstoppable.