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.
The Scarlet Crusade tabard, with its 'authentic' dingy look, is a drop from Scarlet Monastery. If you've never been to SM before, now's the time for a new adventure! The tabard itself drops from Scarlet Trainees who appear after you kill Herod, the boss at the end of the Armory. However, the Armory is locked; unless you obtained the key previously, you're going to have to clear the Library wing too, killing Arcanist Doan and looting the Scarlet Key from his lockbox.
If you're not a stealther, it's a fair bit of work, but fairly easy for any level 60. If you can stealth, or fork out for some invisibility potions, then it's pretty much a piece of cake. Herod is not a difficult fight, but if you unequipped your clothes while stealthing, do remember to put them back on. My first attempt was fairly disastrous as a result of this oversight! The trainees that zerg you after Herod's death have low health, so AOE or dynamite are your friends; the tabard doesn't drop every run, but fairly frequently, and it's bind on pickup (so going in with a group is a gamble).
Enjoy your new wardrobe, but remember -- your guild master spent good money on your tabard design, so don't let your Guild Tabard rot!