In reply to dodfoster:
There are emission controls between states.
This is completely negated if you buy your car in California where they pretty much THE strictest emission laws on the planet. To give you an idea, we brought a Chevy back to the UK from CA and during the emission test, the 3.8l V6 gave out less CO2 than the reader could measure, litterally 0.0 parts per million.
Anyway... yea buy a car in California and then all you have to worry about is a form to fill out when you transfer the title. In the US, each state has a department of motor vehicles, but they are all tied together so its still pretty easy to get around.
Alternatively, you can sell your car on ebay without any hassle at all. The defualt option is for the buyer to take care of both postage and licensing so if you sell your car on ebay, someone will buy it.
Insurance, your probably gonna need a US address (either make one up or meet someone friendly on the plane). Unlike the UK, cars tend to be expensive (even used, way higher resale values) but the Insurance is cheap. So if your staying a short time, rent. If your staying awhile, go mad and get whatever car you want and enjoy the cheap insurance.
Have fun!