Here is the real deal from the Cal. Vehicle Code...
5200 CVC - When two license plates are issued by the DMV 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.
5201 CVC - License plates shall at all times be securely fastened to the vehicle for which they are issued so as to prevent the plates from swinging and shall be mounted in a position to be clearly visible, and shall be maintained in a condition so as to be clearly legible. The front license plate shall be mounted not more than 60 inches from the ground.
For more stimulating reading, here's a link...
http://www.lapdonline.org/bldg_safer_comms/tip_of_the_month/2000/Traffic/00_06_traffic_tip.htm 
