It depends of where you are:
In the UK you seem to be able to get away with an obstructed number plate - guess the cops have better things to do, and figure a car with a load of bikes on the back is unlikely to be driven by a serial law breaker.
It will be interesting to see if this changes once the congestion charging comes in next February in London
(Otherwise there could be a lot of bikes being taken on the back of cars in London every day, as the cameras will not be able to read the plates and therefore they don't have to pay $5 (US$8) toll to drive into the city.)
Elsewhere, such as France or Italy you will stopped immedetely by the Police (and even other motorists trying to do you a favour) should you drive with an obstructed number plate, as many a British tourist has found to their cost.