A full explanation is at: http://www.obd-codes.com/p0420
I think this really is the catalytic convertor acting up. I'm betting it is over 6 years old, as many go out around then. The engine computer is smart enough to know that the upstream (of the cat convertor exhaust flow) O2 sensor is oscillating as it should, and the downstream O2 sensor is oscillating too much, when the downstream one should be flat-lining if the catalytic convertor was doing its job. Tell a mechanic what code you got.
Importantly, you might not have to pay for it, as the EPA requires all auto manufacturers to repair emissions stuff like the cat convertor for free under warranty ( http://www.epa.gov/oms/consumer/warr95f
s.txt ). Warrantied to 80,000 miles or 8 years.