Canuck: The drainpipe for the disposal is indeed above the drain pipe in the wall, by about 4". Although the drain pipe in the wall does slope the wrong way slightly for about 6-8". That would make sense to me as well, except that when the waist pipe in the wall takes a 90 straight down there's still standing water.

YukYuk: My first thought in all of this was that it was a venting issue, so I installed an Oatey Air Admittance Valve to try and bypass the building's venting--and the water still stood still.

Is that not an accurate test? Would the building's ventilation being clogged up still stop the water even though there's an AAV inline?

In any case, I think you're right that if the problem is more than 10' below my floor, and I don't have any living space of my own down there, it's a building issue. I'll give them a call and let you know what happens! Thanks a lot.