The brain working to sort out a conversation with no body language input is certainly doable, but if I had to guess I'd say that's what causes the 1000-yd stare while you're on a cell phone and driving. When I'm conversing with someone actually in the car, I am watching them talk with their hands out of the corner of my eye, but we are also taking in the same scenery around us, and my focus is much closer to the action around my car.

FWIW, too, I can ALWAYS tell when one of the execs in my department is hurtling down a highway in a rented car talking on his Blackberry in a conference call, because he is NOT fully present for the conversations. There's a lot of "whats?" and repeats and confusion etc. Why call in at all, if I just have to type you a memo about the gist of the meeting so you can read it in your BVDs in the hotel? What a waste of everyone's time.

Maybe I wouldn't understand, though, because I never talk on the phone while driving (I hate talking on the phone anyway).