Without getting into a quantitative balance sheet analysis I personally can't see how UAL can survive in the future unless they go through some MAJOR structural changes in management, operations, and labor/management relations. They are so large and regimented that they can't seem to adapt to change quickly enough to accommodate unforeseen economic pressures (not just speaking of 9/11).
Unfortunately, I think this is the inevitable fate of any "old school" company that grows so large. Kind of like "hardining of the arteries". Survival is possible, but only through painful restructuring (IBM and Sears come to mind). All those low cost carriers like JB, SWA, etc, are still small enough to flex against pressure. And when they do increase in size they need to work hard to maintain their operating principles that kept them healthy.
UAL and DAL remind me of "full service stock brokers" whereas JB and SWA are the discount brokers. Most of my friends, including myself (average middle-class Americans), go for what ever is cheaper as long as it's not unreasonably inconvenient and gets the job done. For me meals and seat reservations can't justify and extra $200-300.
I do hope they pull through, though.