People who talk on their phones while in the checkout line is my biggest cell phone-related pet peeve. I've been in line behind someone doing that and I've been behind the register waiting on someone doing it. There were times when I worked retail where the person was gabbing away on their phone and completely ignoring me and I couldn't finish ringing them up because I needed to ask them a question. Then they had the nerve to act annoyed when I needed them to give me their attention. Obviously, you just have to grin and take it, but I was always tempted to yank the cell out of their hands and chuck it across the room.
We had a member here years ago who was, I think, a bank teller. Her tactic if a customer was on their cell phone was to say something like (and nicely, btw) "You know, that looks like a really important call and I don't want to interrupt you, so I'm going to help this gentleman behind you so you can finish up that phone call without me disturbing you." I thought it was a great tactic, although I never had the nerve to try it.