Galstaff of Durotan believes so, and put his money where his mouth is, so to speak, by doing it. (Warning, the movie is 20 minutes long.) While anyone can 'tank' Shade of Aran, I was pretty impressed by his tanking of Malchezzar, especially when he admits in the thread (and his armory backs him up) that while he's achieved immunity to critical hits by a combination of resilience and defense, he's still taking crushing blows. He also says he chugs Ironshield Potions which makes sense, as his armor is low for the instance (in my opinion as a tank, anyway) which is not surprising since he's in mail. He claims 17.7k health for the Prince Malchezzar attempt, which is certainly within the ballpark. (I've done it with about 1k less on my warrior, although nowadays I'd never have less than 18.5 k for any boss.)
His talent selection shows you the limits of Shaman tanking at this time: He has full Shield Spec, Toughness and Anticipation, giving him the most armor and dodge he could possibly have, but by taking Spirit Weapons (necessary for the ability to Parry, which is essential for tanking) he reduces his melee threat by 30%. This is a great talent choice for a melee DPSer who might get a boss turning to hit him before the tank can pick it up. But for a tanking shaman, it means he has to rely on his spell damage (Frost Shock in particular) as well as Lightning Shield to generate threat, with Stormstrike and Windfury doing the rest of the work. Galstaff also mentions that he had to stack the group with a Shadow Priest to regenerate mana since he couldn't use Water Shield: I'd expect he had to use Shamanistic Rage every single time it was up for both the mana regen and the damage reduction.
Basically, as he himself admits, shamans are the worst 'off tank' with no ability to get enough avoidance to push crushing blows off of the table, so to do this you'd need a ton of resilience on mail (since you're not going to find mail with the stats you need and defense) and some very, very good healers. And as long as shaman tanks require a shadow priest to regain enough mana to tank, it's not going to be a common sight. But as a warrior and shaman player, I liked seeing it. Maybe we could get shamans some actual tanking talents, and they could move into the ranks of the other hybrid classes as being able to DPS, heal and tank? If it's good enough for paladins and druids, why not shamans?