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).