UA and perhaps Midwest Express are still in danger, AA
may be in the next year. Northwest and Continental are probably safe, as is Delta(Though is is having some problems now). ATA may be in danger if there is another downturn.
Good times can kill airlines just like bad times can, if competitors are strong and the airline is very weak to begin with(That is, when most assets already have loans against them). When times are good, aircraft and other assets rise in value. In bankruptcy court, it is the creditors and judge who decide if and how an airline keeps operating. When asset values are high, the liquidation option looks more attractive.