California isn't exempt from the clean air act, but rather has the unique distinction of being able set emissions standards that are more stringent than the Federal standards. The other 49 states can elect to use California standards but may not create their own standards.
I'm not sure about the rules regarding importation of post 1973 vehicles, but I'd be very careful to thoroughly research the issue. At present, we don't have any smog check for diesels, but this does not mean that diesel vehicles aren't required to meet emissions standards. It just means that the state doesn't bother to check up on vehicles in service. Thus, even if you get a car imported and registered initially you can be in danger of future problems if the DMV ever decides that your paperwork doesn't show that the car meets standards.
This situation happened to a friend of mine with an '87 Citroen CX gasser. This was a grey market car (as were all CXs brought to the US), and had been in Calfornia for many years before my friend bought it. It had been legally registered and passed smog, etc., but then the owner put it in storage for awhile and didn't register it during that time. My friend had some hassle initially registering the car, but he had the old California registration paperwork and it passed smog, so he thought he had everything taken care of. For two years his wife happily drove it, until it was time for another smog check. Then the computer in Sacramento wouldn't recognize the car's VIN as being acceptable, so the smog shop couldn't check it.
My friend went to the DMV and got told that they had decided that the old information in their files about this paticular CX (and a lot of other grey market cars) didn't show that the car ever officially complied with California emissions standards, so the owner would have to get copies of all the original certification papers from the manufacturer. Since Citroen didn't ever certify the car for sale in the US or California, they obviously couldn't help. The company that had certified his car (and a lot of other CXs) was now out of business, so that was also a dead end. My friend finally managed to get a sympathetic person to agree to allow it to be tested once more based on the old registration papers, but he was told that the official DMV file coudn't be altered without all the certification papers, so he'd better not count on re-registering the car in two years. Once the car passed smog again and was licensed my friend drove it for awhile, but eventually sold it to a guy in another state.
Ironically, I know of other CX gasser owners who have never had a problem, but since the DMV's views of legality are based on the paperwork in their files and this varies according the the year of manufacture, year of importation, certifying entity, etc., it is hard to generalize. So long as we don't have to smog check diesels we are probably safe, but who knows how long this will last.