the college kids around here are always on the cellphone, but I have to say I haven't seen much usage on the bike. I don't really see much problem with it under many conditions. Although I convinced my wife it was pretty annoying after she called me twice while I was climbing the same hill.
Pedestrians on cellphones is a real problem. We have jaywalkers everywhere, and they often just walk out into the street without even looking. Maybe some cyclists do the same, I just haven't seen it.
I just had a bad experience while I was driving a car recently. I was returning a rental, had just spotted the rental car return across 5 lanes of traffic. There was a cell phone user to my left who happened to be steering a car behind me, but as I slowed down, she slowed down shadowing me at half a car length behind. I had my blinker on, she didn't. Turned out she wanted to go right, I wanted to go left. But because of the phone she was immobilized. Fortunately there wasn't much traffic.