The ink is barely dry in the new 2011 code book and we have this gem:
Vehicle Code Section 5201. 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, shall be mounted in a position so as to be clearly visible, and so that the characters are upright and display from left to right, and shall be maintained in a condition so as to be clearly legible. The rear license plate shall be mounted not less than 12 inches nor more than 60 inches from the ground, and the front license plate shall be mounted not more than 60 inches from the ground ...
www.dmv.ca.gov/pubs/vctop/d03/vc5201.htm
California builders should keep this in mind when constructing new bikes for your customers. Please make this information available and it wouldn't hurt to start stocking horizontal license brackets to help them to get into compliance.
Be sure to pay attention to axle mounted plates, they are likely less than 12 inches from the ground.