I wouldn't consider this to be an exploit, but it is a pretty awesome cosmetic trick.
In the Pre-Cataclysm events for horde (I'm sure there is a similar quest for alliance) you get the "Spreading the Word" quest which gives you the "Doomsday Message" item in your inventory that you are supposed to use at all the zeppelin towers. The item puts a buff on your character that puts a giant double sided board on your chest and back with writing on it. This buff DOES persist through death, and the item doesn't take up any equipment slots. It looks like if you DO NOT finish the quest, you get to keep the item indefinitely!
There is no "expiration" on the item, and it is currently unknown if the dev's are going to take it out of the game after Cataclysm releases. Since the quest really doesn't give you any reward at the end, why not keep a giant sign on your chest for now on letting everyone know how awesome you are?