The problem with going BK nowadays is that no one wants to throw money into an airline when it's pretty obvious that the whole industry can't cope with fuel prices like they are now.
UAL could be in the same boat by the end of the year. Not much left to mortgage and little prospects for getting any kind of profit in the future. Not a whole lot of lenders who see that as a good risk.
The airline industry is in a high stakes game of "last man standing."