Aloha was hurting for a while. GO airlines really ate their lunch in interisland travel.
ATA I have no idea, but I am guessing they couldnt stand the gas.
Frontier Airlines is using bankruptcy to renegociate with lenders and credit card companies, they say they will come out just fine. We will see.
Everybody else is just too small to stand the heat of gas and competion. There was an article today about how some are speculating that we will emerge with 3-4 superairlines instead of 7 large airlines and a whole bunch of little upstarts.

Expect alot of layoffs in the next few years, and I wouldnt be suprised to see some of the pensions get offloaded to the Gov if they havnt already. It is fly cheap or die and labor is still the biggest single expense of the airline industry. That being said, Southwest isnt much of a bargin anymore. I use to fly them alot cause they were cheap. Now, I can usually fly some big name carrier instead at equal or cheaper price.