I've been toying with the same idea; a Death Knight + Pocket Healer. At the moment I'm leveling an Enhancement Shaman (melee) and a Resto/Balance Druid. I'm only at lvl 16 and haven't entered a BG yet, so I don't have enough experience to comment on the viability. However I can imagine the instant heals, the CC and the stealth of the Druid will be very effective in PvP. Is a Priest better? I dunno.