A New York Post article talks about the growing popularity of cellphone jammers. The devices, which are illegal but can be bought on the street in New York City, are giving relief to people sick of loud-mouthed cellphone users. They seem to be especially popular with users of public transportation, where you don't necessarily want to listen to the details of someone's love life while taking the train into Grand Central Station.
Some of the jammers are no bigger than a cellphone, and operate with a radius of fifty feet--just right to cut off the chatter in a bus or train car.
I think we're still in the novelty phase of cellphones, in which appropriate use and etiquette are evolving more slowly than the constant increase in cellphone users. Ten years ago, a similar debate was taking place about the incivility online in Usenet groups and discussion forums. Today, you don't see much worry about flame wars and rudeness. It's not that rudeness and namecalling have disappeared; it is just that most people know to ignore such behavior.
Over the next year or two, I think many of us will turn our phones off more often and become less accessible as the novelty factor wears off and we realize that there is more to life than walking around with the phone glued to our ear.