I think August is the most recent month for which information is available from the Department of Transportation...

http://airconsumer.ost.dot.gov/repor...200810ATCR.pdf

AUGUST
CONSUMER COMPLAINTS: RANKINGS/U.S. AIRLINES * (complaints per 100,000 passengers)
1 SOUTHWEST AIRLINES 0.22
2 ALASKA AIRLINES 0.31
3 EXPRESSJET AIRLINES 0.41
4 SKYWEST AIRLINES 0.42
5 PINNACLE AIRLINES 0.57
6 NORTHWEST AIRLINES 0.73
7 FRONTIER AIRLINES 0.79
8 MESA AIRLINES 0.92
9 CONTINENTAL AIRLINES 1.09
10 AMERICAN AIRLINES 1.22
11 ATLANTIC SOUTHEAST AIRLINES 1.23
12 AMERICAN EAGLE AIRLINES 1.30
13 AIRTRAN AIRWAYS 1.37
14 DELTA AIR LINES 1.79
15 HAWAIIAN AIRLINES 1.79
16 UNITED AIRLINES 1.84
17 COMAIR 2.07
18 JETBLUE AIRWAYS 2.19
19 US AIRWAYS 2.45