The last time I checked, AirTran, ACA, and Skywest were all accepting app.s with plans to do hiring later this year. I don't know if that has changed lately. Comair is actively hiring although their classes are a little backed up due to the Bombardier strike. With the strike being cleared up, things should be back on track. One of my co-workers is headed their way in July.
Comair does a lot of preferential hiring. They try to take on most of their Academy instructors of which I was one of not so long ago. The Academy side is horrible as far as pay goes, but on the airline side you'll see nothing but smiles (esp. with the new contract). In addition to the Academy, Comair absorbed a number of the Midway guys, I think they're also helping out the poor guys in the USAirways Express system thanks to LOA 81. (Could be wrong about the last part though.) It can be relatively difficult to get into, but keep trying.
Of all the regionals, I believe ACA is hiring the most. When I went by pilot recruiting a month ago, the board said they were hoping to hire 320 this year. Two week ago, I stopped by and it had been upped to projecting 370 this year. To all those interested in ACA: keep updating. Knowing somebody here helps, but an interview captain told me we are interviewing 3 days/week at present, soon to be 4 days/week, so keep on sending your stuff in and keep your fingers crossed.
Of the other regionals, I believe SkyWest is doing hiring, though I hear they prefer people with 135/121 or 'mountain' or 'west of mississippi' flying. Comair was hiring, they may have slowed down. Look for ASA to possibly pick things up later this year. I believe they are interviewing right now.
I was pretty sure they had stopped interviewing with people currently sitting in their pool. Some have been sent update packets but NO hiring right now. Therefore, none of the major pax airlines are hiring.