I play on a number of servers, and a fashion is spreading through them -- players are donning new tabards everywhere from Ahn'Qiraj to Zul'Gurub. The two items, Tabard of the Argent Dawn (left) and Tabard of the Scarlet Crusade, surfaced in the last patch.
The Argent Dawn tabard is obtainable as a result of the invasion event. Initially, you have to take ten Necrotic Runes to the Argent Quartermaster at Light's Hope Chapel. Finishing this quest will activate an Argent Quartermaster, and you can then trade more Necrotic Runes for items like mana potions and Consecrated Sharpening Stones (+AP against undead). For ten runes, the tabard is yours. I found this tedious to solo, but when grouped with a mage and a couple of others the runes flow heavily.