I've had one on the front of mine since May, i figured with just the front they wont spot it unless they're coming towards me and i'm hoping no copper is going to bother turning round for the sake of a number plate. I've stuck it on over the proper one with double sided pads as:- (here's a word of warning)
Several years ago a mate of mine had itallic lettering on the number plates of his RS turbo, he got pulled over regularly and got off with loads of warnings but on one occasion the copper ripped off his no. plates on the grounds that they were not legal (bit red-neck america if you ask me) breaking the front bumper in the process. he then fined my mate the £30 someone else in the thread mentioned, and threw in a producer for good measure!!! naturally my mate put in a complaint about this blatant abuse of the coppers authority which got him nowhere, but then he is ginger... (sorry gingers!!!)