Cops don't normally go out of their way to bust people for not having the front plate on the car but it is illegal especially because of the redlight cameras. The actual language for the CA vehicle code for 2003 is after this.
http://www.dmv.ca.gov/pubs/vctop/d03/vc5200.htm
5200. When two license plates are issued by the department for a vehicle, they shall be attached to the vehicle for which they were issued, one in the front and the other in the rear. When one license plate is issued for use upon a vehicle, it shall be attached to the rear thereof.