Three players, two Death Knights and a Priest, have recently trio'd almost the entirety of Karazhan, minus only Nightbane due to not having completed the quest needed to summon him. Admittedly both Death Knights were using a deep Unholy build which is tuned a little high now, but they're not wearing any high end gear, either. The spec that the tank used is a bit unusual for an Unholy Tanking build. They didn't take Bone Shield, and I suspect Magic Suppression being bugged had something to do with its exclusion, though that may have been fixed and I missed it. A fun little detail they discovered is that Gift of Arthas, which procs a disease, is counted as one of your diseases as a Death Knight and augments abilities that scale depending on how many diseases you have active such as Death Strike.
So there's your answer, ladies and gentlemen. Karazhan can be 3-manned before any raid gear is available, so Burning Crusade heroics will surely be able to be duo'd if you're still hoping and praying for Anzu come level 80.
[ via DeathKnight.info ]