Finwe pointed out that if you are leveling a new Priest as an alt, and you have a level 80 main, you should pick up some Heirloom items if you have the Emblems to spare. If you are not familiar with the relatively new Heirloom Item system, here's the skinny. These items bind on account, meaning you can mail them from your main to your alt, as long as both toons are on the same server. The stats for the item change depending on the level of the toon carrying the item, and are usually approximately the same as a Rare item of the toon's level. You can enchant these items, but the enchant cannot have any level restrictions, or you will not gain any benefit from them.

Here is a listing of all the available BoA/Heirloom items via WoWhead. There are lots of good PvP items as well - these must be purchased from Knight Dameron (Alliance only) or Stone Guard Mukar (Horde only) when your faction controls Wintergrasp, and are purchased with Stone Keeper's Shards. Here are my selections of good items for a leveling priest - or any cloth-wearing, non-PvP caster.

• Tattered Dreadmist Robe - (Chest) Cost: 40 Emblems of Heroism (remember you can change Emblems of Conquest to Valor, and then to Heroism) or 60 Champion's Seal. These robes give a 10% bonus to XP gained from killing mobs. Enchant with Greater Stats or Major Mana.
  
• Tattered Dreadmist Mantle - (Shoulders) Cost: 40 Emblems of Heroism or 60 Champion's Seal. This shoulder piece gives a 10% bonus to XP gained from killing mobs. This does stack with the robes, for a 20% bonus.
• Devout Aurastone Hammer - (1-hand Mace) Cost: 50 Emblems of Heroism or 75 Champion's Seal. One handed mace, you can change the off hand item to match by picking nice items up in the AH as you level. Enchant with: Spellpower, Healing Power (both of these recipes are difficult to find because they drop in Molten Core) or Mighty Intellect or Mighty Spirit (still tricky to find, but slightly more common, requires rep with Thorium Brotherhood and a trip to Blackrock Depths for the Enchanter to purchase the recipe).
  
• Dignified Headmaster's Charge - (Staff) Cost: 65 Emblems of Heroism or 95 Champion's Seal. Good choice if you don't feel like bothering with off-hand items, and want to just stick with a staff. Enchant with Major Spirit or Major Intellect, and if you can't find these, Lesser Spirit or Lesser Intellect.
  
• Discerning Eye of the Beast - (Trinket) Cost: 50 Emblems of Heroism or 75 Champion's Seal. Awesome trinket - especially since you usually don'e get your first trinket until somewhere in the level 30-40 range.
• Swift Hand of Justice - (Trinket) Cost: 50 Emblems of Heroism or 75 Champion's Seal. Another good trinket. After all, you get to have two. If you have to make a choice between them for your priest though, I recommend the previous one.

Items purchased with Emblems of Heroism can be purchased from a couple of places:

• Brammold Deepmine in Dalaran.
• Enchanter Erodin in Dalaran (HORDE ONLY)
• Enchanter Isian in Dalaran (ALLIANCE ONLY)

Please note that if you need to convert your Emblems of Conquest to Valor and then to Heroism in order to purchase these items, I do not think you can do this at Brammold Deepmine - you will need to go to your factions' seller, and convert them via the other vendors nearby - it's a pain in the butt process too.

If you are purchasing Heirloom items with Champion's Seals, you can purchase them from Dame Evniki Kapsalis at the Argent Tournament, located at 69,23 inside the Argent Crusade's tent. Please note: You cannot purchase these items from the Dame until you are an Argent Crusader, meaning you have earned either the Exalted Argent Champion of the Alliance or Exalted Argent Champion of the Horde achievements.