I was told that it has nothing to do with static discharge or RF interference but is for two reasons - firstly it distracts you from the - potentially - dangerous act of filling you vehicle with highly inflammable petrol and secondly, a spark may be caused if the phone were dropped and the battery were dislodged. Petrol vapours hugging the floor could be ignited.
The radio transmission signals from early mobile phones had to be a lot more powerful than they are now because there were not many receiving base stations around.
High energy radio waves can knock electrons out of atoms causing a spark (thats all a spark is - moving electrons). This is very dangerous when highly volatile petrol vapour is present because it could obviously cause an explosion.