I've noticed that talking on a cell phone while driving, for me, is a huge distraction, and I think it is different from having a conversation with somebody who is in the car with you. The reason why is because when you're driving you have stretches where very little effort or concentration is involved, and this is when the conversation with you and your passenger usually is taking place, and when you approach an intersection or other other situations that demand more concentration and decision making, the passenger is of course aware of this and the conversation naturally lulls until you're back to a less demanding situation, where the conversation resumes.
But when you're on a cell phone, the person at the other end doesn't know when you're in a situation that demands extra attention and concentration, and if they're talking away about something or another, you the driver can't help but feel somewhat compelled to pay attention to them, and it is distracting.
And the radio isn't a distraction because you don't feel compelled to give it your undivided attention or any attention at all for that matter, the way you do a phone conversation, so I don't think that comparison is apt.