This situation seems to fit the definition of false imprisonment which most states recognize as any detention of an individual for however a short a time without that person's consent and against his will, whether by actual violence, threats or otherwise. Obviously, no court is going to consider minor delays the equivalent of false imprisonment. There has to be recognition that by purchasing a ticket we have impliedly agreed to some limitation on our freedom. But once we start talking hours, it seems to me a pretty good argument could be made that passengers are clearly being held agains their will. I can't imagine any airline would want to fact such a charge and I would expect DL, like NW, to go to great lengths to mollify these passengers.