Last night, I managed a different variation on that. We were fighting Kel'thuzad. I was main tank. We had an off tank who's relatively new, but shows great promise. He picked up the Guardians of Icecrown when they entered the fight, but some glitch happened between him and his healer, and he dropped like a stone.
I roared and growled and managed to get the two guardians to attack me after they killed only one more player, and we managed to finish the fight with me tanking all three foes. (And sweating bullets doing so.)
Which means that I managed to grab aggro when the tank went down and finish the fight--when I was the main tank.