Er...your comparison is not exactly correct. One cannot compare speaking and listening to music while driving similar to using cell phones and signing. This is not to say that the former activities couldn't be distracting to the driver. Of course, this could be the case. However in general, this is not true. Otherwise, almost everybody driving would be driving erratically as most people are either talking to somebody or listening to the radio while driving. Heck, I do it all the time and have no problem keeping my attention on the road!
The issue here is that it has been observed that using cell phones often lead people driving rather erratically and could and have led to dangerous situations (i.e., accidents). Why would using a cell phone be any different than just speaking to a passenger? There is something about using a phone that forces one to focus on the phone itself and less on what is around you. That is one reason why I don't use a cell phone while driving (except real emergencies). I did it once in the parking lot going very slowly and I couldn't believe how distracting it was! I don't know how others do it.
In the case of signing, I can see that one's attention is definitely diverted as you need to use your eyes to see what's being said. That often means turning your head to the signer and it is obviously not a good thing if you are the driver. Some of you in this thread mentioned the difficulties of signing while being the driver.
Whether or not they need a ban on signing while driving...I dunno as the jury still out on that one plus there aren't that many drivers who sign. I do agree with the need to ban the use of cell phones while driving except in emergencies. The problem I see with such a law how do you enforce it? You got cell phones that have attachments to sit on the ear and it is difficult to see them. About the only way people really get caught is either driving like an idiot or it is very obvious they have a cell phone in their hand.