KPSB and KUNV are about 13 miles apart, both have the same runway configuration: 16/34 and 6/24, and both are uncontrolled. I'd guess that on that route of flight, PSB came along first and probably lined-up pretty well with where UNV was supposed to be, even if it was a little early. If it was a visual approach (and I GOTTA think it was...), an airport showing up in pretty much the right neighborhood with the proper numbers on the ends of the runways might present a tempting target.
However, both of these airports have an ILS and there is a VOR roughly between them, so it wasn't really like visual ID was the only way. I'll be interested to hear the particulars. Pity they didn't see their error before they touched down. Seems like a classic, "there but for the grace of God..." event to me.
Dave
PS (FYI) Shuttle America shows up in the FAA database as a Part 121 operation with 20 SF-340s, DBA USAIRWAYS Express. Six years in business and based in Ft. Wayne, Indiana.