California, the bastion of state governmental
interference regulation, will see compliance to the Transport Refrigeration Unit (TRU) updated to include all model year 2005 and older units as of 12/31/2012. The regulation has been in existence since 2004 but the end of year year sees newer models fall under the regulation.
For a better understanding of this regulation read the complete story at the California EPA website.
The basic gist is that the reefer units on all vehicles that pass through California must demonstrate adherence to the regulation. Compliance will require registering your TRU with the California Air Resources Board who will award you with an IDN that must be affixed to both sides of the TRU housing.
Both Carrier Transicold and ThermoKing have compliance assistance documents available to help find the information necessary to complete the required forms.
More regulation is not what anyone wants but keeping updated and on the right side of them always matters. If you run reefer and you pass through California then make sure you know whether your rig is in compliance and what you need to do to either get it compliant or prove that it is. Read the EPA article linked to above to get the complete story.