But I'm sure it's only a matter of time before someone loses it and decides to pop an exit hatch and escape his "captors." Once I spent 3 hours on the ground at DFW with no A/C and I can tell you that it sure crossed my mind. Luckily I was still sane enough to think of what a royal mess that would have caused and how long I would probably have spent in jail.
You can be fairly well assured of spending time in jail in a case like that. A passenger who has paid to be there may have a very hard time making the case that he or she is a "captive." Further, creating such a potential emergency, enciting panic, and the possibility of someone going out a door and into an engine or becomeing hurt or killed in any number of ways could put one in prison not just for a short term, but for a lifetime. In fact, simply mentioning such an idea, even jokingly, would warrant removal from the aircraft and possible arrest.
Sitting tight during delays isn't much fun. It's no fun for the crew any more than the passengers. Take a good book, close your eyes and sleep, wear an ipod, or do something to occupy yourself.
Lack of air circulation should be an issue. If the delays have been long enough to warrant a shutdown, then APU air should be more than sufficient to run at least one air conditioning pack on the ground.