Ten Ton Hammer has a PvP guide up this morning for the Priest class. Please visit the source for the entire guide, but here's a couple of snippets that are worth highlighting.

Builds

The first thing we need to look at is how we are specced. Our spec, more than any other class, will dictate our role in PvP. A Shadow Priest is going to do very well one on one in PvP, while a Holy Priest has no chance in a fight and must take a support role and heal his teammates. A Discipline Priest can do a little of both depending upon what’s needed, but your most important quality as a Discipline Priest is survivability.

Gear

Your priorities change in PvP. Your gear has to change too. In PvP we are introduced to a new stat. Resilience, Resilience reduces the chances of you receiving a critical strike or critical spell effect. In addition, it reduces the damage taken from criticals and damage over time spells. The only way you can increase resilience is through gear, enchantments, gems, flasks, or elixirs. It is the most important stat in PvP, greatly slowing down combat and giving you some breathing room to think during the hectic fights. Spirit is almost meaningless now, and Stamina becomes more important. Stack as much Resilience and Stamina as you can with + Spell Power and + Intellect following. Remember your fights will be over quickly, with little to no down time. You have to be ready to take a few hits, and unleash as much healing or damage as you can as fast as you can.