In the UK it surely doesn't help at all that many people say 'Oh' for both the lettter 'O' and the number '0' - My wife's car starts RO09 which many would say as 'ar, oh, oh, nine'.
Apply that same thing to the minds of those individuals who may not be very technical, nor very good at remembering their own registration plate - they'll verbalise it in their head and quite possibly use Os instead of 0s or vice-versa.
I strongly believe that people should never be punished for making a mistake because of a badly designed interface.