Yeah, this is the major flaw in the "I can multitask better than most people so it's OK if I do it" argument. What's dangerous about driving is other people. It's not a question of whether you can drive in a normal situation and talk on the phone. It's a question of whether you're going to be able to react in a split second if some idiot who isn't looking where they're going does something totally unpredicatable (probably because they are talking on the phone). You need ALL your faculties in that case, and if you're talking on the phone, all your faculties are not available. They're just not, no matter how smart you are.
I realized today that part of what drives me crazy is that I don't understand why people don't intuitively get the perceptual problem. Even if I do something as simple as change the settings on the radio without taking my eyes off the road, I can actually feel the tunnel vision thing happening. You can't visualize where the radio knob is and at the same time see what's on the road in front of you, because some of the same parts of the brain are being used. If you're driving down a familiar road and nothing unusual happens, you can get away with it, is all.