Magtheridon, once one of the hardest raid encounters in the game (depending on who you talk to), has been 4-manned by an Unholy Death Knight (who you may have read about previously), a Retribution Paladin, a Restoration Druid and a Holy Priest. If you've done the fight before, you might be thinking to yourself, "That's not possible. You need more people than that just to click the cubes!" Apparently that is not the case. They didn't bother with the cubes, they just healed through the damage.
They healed through a lot more than that, too. They didn't have the ability to stop the heals that the summoners cast, at least not all of them, so they weren't able to kill them at all. The Death Knight tanked all of it: Magtheridon, the summoners, the summoned abyssals, the whole deal.
Completely awesome, and I'm sad that there's no video. With the way WoW scales, I figured it was only a matter of time before Burning Crusade raid content (beyond Karazhan) started getting 5-manned, and I imagine this is only the beginning. I'm looking forward to seeing a Death Knight riding the Ashes of Al'ar. A Death Knight riding a Phoenix is so metal it hurts, and is also somehow appropriate in that weird kind of way.
You can find a few more details right over in the DeathKnight.info forum thread made by hadeniaz, the Death Knight himself.