How worried should passengers be about the United/Continental computer switch?

Not very, but when two airlines combine their computer systems, as the merged United and Continental are doing late tonight, nasty stuff can happen. From USA Today:

When US Airways and America West combined their systems in 2007, the computers failed to communicate with airport kiosks, causing long lines and delayed flights. Customers had trouble making and changing reservations when Virgin America moved to a new system last fall.

Actually, the Virgin America snags have dragged out for months. These systems are immensely complicated. Last month, Businessweek had an interesting piece about the many puzzle parts that go into combining two large airlines. On the computer switch:

Throughout the new United, everyone is talking about a single passenger service system--PSS, they call it--in somewhat apocalyptic tones, the way IT consultants once invoked Y2K. As with the flight system, there are technical issues--the company isn't eager to repeat US Airways' debacle--but there's also a human factor. The new United is adopting Continental's passenger services system, a Hewlett-Packard (HPQ) program called Shares. According to Martin Hand, United's senior vice-president for customer experience, the program is more flexible than legacy United's program, Apollo. Shares is easier, among other things, to customize so it can ask travelers whether they'd like to purchase an upgrade or extra legroom. But Shares is also less intuitive than Apollo, and United veterans are struggling to learn it. According to Smisek, all the dry runs have gone well; just after New Year's Day, legacy United agents handled all the Continental flights at LAX.