A CVN is a computed checksum of the calibration data inside the ecu (the tune), its calculated every time the ECU is switched on, when the manufacturer issues an official CARB/EPA firmware they provide the CVN along with the testing information. Its then recorded by CARB on their centralised database, if the number read from the car in a smog test is different, its a fail. However as i said i've never heard of anyone failing that test unless they went to the referee.
The information about your car is fed to the state though, so that numbers on there.
Everyone pretty much gets away with it, most places just pass you regardless. Half the problem is no doubt the government can't get its act together and build a properly networked system and some of the smog shops are just too far behind technically to handle it. But the advanced newer shops are networked and can just do a PASS/FAIL, the EPA just has to flip that switch.
Remember its an EPA law, so federal not state.
Also if you get caught with an o2 simulator or ecu simulator thats designed to defraud any of the emissions stuff, its upto a $2,700 federal fine (plus any local/state fines) for the owner of the car, and a much larger one for whomever sold it.
Turning off the MIL light is one thing, making the ecu appear like its passed, another.
Its also illegal to even offer them for sale, and if you think the EPA never does anything about it, talk to caspers electronics see what they say about it, that is if he's still in business.
as said most people get away with it, but i reckon its always best to know the worst case.
Black Exige S / Elan M100. Don't run a smaller pulley without an upgraded fuel pump! http://www.goth.am
ecu stuff.. New reflash box coming soon!