Often in these situations I'm unfortunate enough to have decided to DPS on a character that can tank, but luckily (or unluckily as the case may be) yesterday I was on my shaman and so could simply sit back and wait. It was mentioned to the 'tank' that, as the person who had signed up to tank the instance it might behoove him (or possibly her, hard to tell who's behind the keyboard) to slap on a shield and some tanking gear, but only the grim silence of the truly disinterested was our reply. Was he even at the keys? Hard to say. The wait extended to the point where we could safely kick him, and kick him we did. but that's ten minutes or so of my life I'll never get back. (And no, my shaman couldn't tank HHoR, I didn't even try).
This is leaving out the fun of the HFoS run where the 'healer' was in full Elemental gear, used Flametongue on his weapon, and let the melee die twice on trash pulls because the only healing spell he cast was Lesser Healing Wave, only to roll need on Nighttime and drop group immediately. At least he pretended, I guess. (Best quote of the evening was from the surprisingly friendly and helpful death knight... sorry, dude, but it goes against your class' whole deal... who said "You went pretty much that whole boss fight with no healer and we still killed him" in front of the guy.)
So, I ask you: do you queue as a tank or healer to get a faster group and then hope someone else will do it? Or queue and then sort of half-heartedly pretend? Or have you run into one of these folks and found yourself tanking or healing a run you signed up to DPS?