From what I've heard, BAR was doing this all last year, as well. There's just no way to prove it now. The FIA was tipped off by at least one former BAR employee. Rumored to be David Richards!!! I wonder if F1 were fielding 24 cars each race, if the penalty would have been more severe. They were using a 'hidden' fuel tank that allowed the car to run underweight at various times during the race, but never at the end of the race. When the car was drained of fuel for post race inspection, the hidden tank was never drained. That is, until Imola when the FIA was on the look out for it. They 'hoovered' the fuel out of the tank. Clearly running under weight or longer between refueling would give the car and advantage. No wonder Jense wanted to get away from BAR at the end of last season.
