Just my personal opinion, but I think it's the height of rudeness and poor manners to use a cell phone when your transacting other business in a supermarket or bank etc.
It's a complete insult not giving people who are helping you, to give your full attention.

Face to face interaction in any social situation warrants a much higher priority in the attention stakes than a gadget.

Fair enough if someone calls and it may be important, but surely make sure it's possible to excuse yourself with an apology then move off to a more private location.