The problem seems to have becopme worse of late, perhaps it's because there is a whole new generation of drivers who were brought up with the mobile phone as an essential piece of equipment.
What I find perhaps even more disconcerting than car drivers with a phone stuck to their ear, is someone wrestling a 10 tone lorry one handed round a busy roundabout whilst holding the phone to his ear with the other, this I observed recently.
Unfortunately like everything else, they always take the cure too far, I'm sure that the insistence on full car kits being used has nothing to do with the government trying to give something back to the mobile phone companies after the 3G bidding fiasco.