As an unannounced change in this morning's patch 3.1.1, Exorcism can no longer be used against players. The stated rationale for this change not being announced ahead of time was that they were "trying to get the tooltip changed at the same time to reduce confusion," which doesn't quite seem to hold water for me - wouldn't it be less confusing to tell the players, and not leave them to find out for themselves the next time they tried to PvP? Anyway, that's a relatively minor issue.
Recall that in patch 3.1, Exorcism was changed to be usable on all targets, and to automatically crit against demons and undead (before, it could only be used against demons and undead). Evidently Blizzard feels that it's just too much burst damage for Ret pallies in PvP, especially given that it means they can start up their combos from range. Edit: Just to clarify, the functionality of Exorcism in PvE is unchanged.
Ghostcrawler also promises that they're working on making Pally DPS "more interesting and also less bursty" instead of just using all your skills on cooldown, and that once they figure out how to do that, you'll be able to Exorcise other players again. He gives Conflagrate, Brain Freeze, Rip, Overpower, and Arcane Blast as examples of the kind of more interesting mechanics they might be going for. He invites feedback on ways to make Pally DPS more reactive, so go make your thoughts known in the thread if you have any ideas.