Maybe not worse but at least as bad. This was the subject of a speech & position paper I did in college. People who were driving and talking on a phone were just as dangerous as drunken drivers according to the research that I found. I usually commute on a motorcycle and I am always on the lookout for yakkers.
Not that Mythbusters are a definitive answer to anything. But watching the show where they compared talking on the phone (yes, even hands free) vs. .08 driving. Cell phone usage was much more dangerous. A distracted driver, is a distracted driver. A driver can not concentrate on the road when they are concentrating on having a conversation.