In reply to Paul Winder:
I did a similar thing, also from San Fran and it's really no big deal. Buy in the same way that you do in the UK and the transfer to the new owner is very similar - you get the slip and the dealer has to post off the main portion to get you as the owner.
Insurance was 200 quid per person for 6 months (I was there for the ski season so didn't need it any longer than that, it may be cheaper for 12 months.) I can't for the life of me remember the company but it was basically a flat rate for a foreign license driver and there weren't many questions asked beyond the normal.
Cars aren't cheap in the states, unless you're looking to buy an absolute dog. We got a '93 Ford Explorer and it was around 3 grand. We sold it back 6 months later for 1,700 but it'd had a hard 6 months sliding around Tahoe.
In Cali, you need a smog certificate and it's illegal to sell a car without one, so make sure you're not getting shafted on that. We almost did. They have an MOT equivalent as well but it varies from state to state and I don't know too much about it because we couldn't be fscked with it.
If you're driving on snow, get snow tyres. 4x4 you can do without, snow tyres you will need.
American cars vary in reliability from very to extremely unreliable. Get a Japanese car if you can.
I miss the car.