I've got the same problem. I talked to the dealer a month ago and they told me that it was probably a gas cap problem. Mine was fishy, because the cap was missing the pressure seal in the middle when I got the car used last year. I went to Autozone and got an aftermarket one for like $4.
The problem persisted, and today I went to the dealer for an oil change. They wanted $45 to check the code at first, but I talked the manager into just checking it for free

.
First thing: he freaked about the aftermarket gas cap. Said it wouldn't work...dunno why. Told me to return it and made me buy a TOYOTA one for $17. To my uneducated eye, it looks the exact same

. Just a lot more expensive.
Anyways, he checked the code...sure 'nuf it's an O2 sensor code. But he reset the light and told me to come back later if there was a problem. The light didn't come on until I hit 70 MPH on the highway.
I hope to God it's not an O2 sensor. I don't have money
Car seems to be running fine...but I know if the sensor is dead, I'd better fix it soon. I just don't have $500 to dish out.