What about the same spec, but two different sets of gear? I still raid, and I have been doing arenas for a few weeks now. I have a set of raiding gear for when we do kara and PvP gear for when we go do arenas. I don't know if that would work for priests, since I am not a priest but a druid...
I have healed when we raided pre-BC, and I have done both dps and tanking because I am feral now. I have to agree with many of these people, that tanking is the most difficult for me to do, followed by healing, and then dps. I have to say dps is the most fun out of these three though ^^