Southwest wasn't the only app I found that username and password issues. There is a list below. Note that typically users have a really high rate of password reuse so if we are able to compromise one account, the chances are likely to be able to compromise others.
Cloudette: Username in plaintext and password, hashed with MD5
Gas Buddy: Username and password, hashed with MD5
These two apps (Cloudette and Gas Buddy) are mentioned because you could replay these credentials to login to that account.
Southwest Airlines: Username and password in plaintext
Minus: Username and password in plaintext
Wordpress: Username and password in plaintext
Foodspotting: Username and password
ustream: Username and password
Labelbox: Username and password
Of the 253 applications surveyed, 91.7% had no risk found, 3.1% had a low risk, 2.3% had a medium risk and 2.3% had a high risk. While it would be desirable to have no applications in the “Medium” or “High” category, the number of applications the authors found presented a security risk was both surprising and far too numerous. There are over 500,000 applications on the iOS App Store, so extrapolating the results, there could be at least 15,500 applications in the “Low” risk category and 11,500 applications in the “Medium” and “High” risk category.
You can find the full details here: http://blog.afewguyscoding.com/2012/01/affected-applications-a-survey-mobile-device-security-threats-vulnerabilities-defenses/