Peace is right. 25 years old or older. There are certain models of BMW/Land Rover that can be newer and brought back to the US, but you've got look it up on the US Dept. of transportation like Peace linked to.
When my wife & I first got to the UK a salesman over at the car lot by RAFM tried telling us we could send back the 1996 Audi A3 he was trying to sell us. We fell for it and bought the car as we REALLY needed a car for myself. Suckers! We later found out not only did we pay tooooo much for the car but that we couldn't take it back either.
So, for the most part if it's an older Austin Mini or an older Land Rover (or any other old British car) than you're golden. If it's newer than 25 years old....sell it.