i have driven with an authentic German/Stuttgart plate on the front of my car and the registered CA plate on the back for 4 years... no problems aside from 1 fixit ticket from airport police ($10). airport police are instructed to ticket for any and everything they can to cut down on terrorism - they need to have a reason to stop you.
though ive been pulled many times since then with no comment or ticket about the plate. here it seems they don't care as long as you have a valid plate on the back.
the plate i ordered from europeanplates.com looks like the attached image.
you can see my plate from Alex by clicking the link below. it's authentic German replica material. it's awesome! you need to buy the frame holder for it to fit your US screw holes.
http://flickr.com/photos/ekstarr/sets/72157603850739088/