The other problem with cellphones is the potential for distraction, and I'm not even talking about the obvious one with driving and texting.
I live in an area with lots of middle and high school kids, and when the bus drops them off, or I see them in outdoor malls, every single one is walking along staring at that little device. I've seen near misses with autos, I've seen at least one nasty fall when someone tripped on something, and a ton of person-to-person collisions.
And, in the interest of full disclosure, I've been guilty of it myself in a crosswalk. You just can't do that.