I don't know about general distraction studies, but a study done on distracted driving showed that cell phone conversations, even hands-free ones were more distracting than conversations with people in the car. And there are good reasons to expect this result. Anyway.
1. I don't care if it's a luxury, or about people flaunting things. I care that people pay attention, a lot, as a runner and a cyclist.
2. Radios don't demand your attention the way that phone callers do. You can completely tune them out -- often while driving with a CD playing my favorite song on the album will go by and I won't notice until halfway through the next one. They don't demand much of your reasoning abilities, don't demand that you try to remember anything, and are very unlikely to frustrate or annoy you, as callers often do. If radios were anywhere near the distraction problem of phones people would be studying them.
3. Passengers in the car know that you're driving. They don't just know it as a fact, they're in the same boat. They can see traffic conditions and respond to your body language. They know when you don't respond immediately that you're still there (pauses become awkward *fast* on the phone -- someone talking to you face-to-face would never "ping" you by saying, "Hello?" or, "You still there?" like people do on the phone all the time). Passengers, in fact, often help drivers operate the radio/heater/windows, find change for tolls, navigate, and notice dangerous situations on the road. That's not true of everyone, but it's true of just about all my passengers.
I was once almost hit by a woman holding a dog in her lap (I was on my bike, caught up to her at the next stoplight, and knocked on her window to notify her that she was driving unsafely -- she said she didn't see me, which was really scary). I know that pets and children can be a problem... so store them properly! Cats and small dogs have carriers, children have child seats. Believe me, I don't ignore this problem. There's another factor to this -- often when traveling with passengers the whole point of the trip is to get the passengers somewhere. Banning them would defeat the whole purpose.
4. This is a caricature. I am quite aware that there is no one cause for anything. When on the road there's a certain amount of risk that we have to accept. When people willfully do things that introduce risk beyond that we should single those things out. Speeding, bad lane behavior, driving drunk/high, fiddling excessively with stereo/heater/iPod, holding a dog in your lap... I will single those things out. Using a phone to text or make a call is one of those things also; based on actual studies, it increases the risk of an accident more than drinking to the legal limit in the average person.
5. More caricatures.
6. More caricatures. I should note that when you drive distracted you externalize that additional risk you create onto other road users. So you're imposing your will on others as well. When externalities are involved, the argument, "I should just do WTF I want" doesn't hold much weight. True, some people like to tell people what to do when their behavior doesn't really affect anyone else. Anti-cell phone people are not among them.