Mobile phones (or cell phones in USA) are a product of the tremendous technological achievement in Electronics and Communications. There is no denying that such a thing would have been laughed at had it been proposed about 40-50 years ago. But it is a reality and, with ultra-cheap silicon technology, has made a grand foray into third world consumers' pockets. No one would have imagined that a country like India, where the penetration of land lines was abysmal just 10 years ago, would see pani puri wallahs and road side workers wielding mobile phones.
With new technolgy and new lifestyles come new problems that people don't know how to handle. No one has ever been instructed about how to use these mobile phones in public. There are no set rules on etiquette or accepted norms of behaviour regarding them. So what is stopping people from hollering into their phones in a crowded bus or on the street? Nothing! People just do not care about neighbours being bothered or other people trying to concentrate on their work.
We have had mobile phones thrust upon us for almost no cost. So now, whether or not we have anything to talk, we cannot stop taking. Conversations can go aimlessly or discussing your most personal thoughts and feelings in front of total strangers who are made to put up with your agonies. More irritating are the boisterous types who think that the neighbours have nothing better to do than listen to your drivel; and they go on talking at the top of their voices to the utter consternation of everyone in the room.
This even happens in friends' circles where the idiots cannot go a little distance away to speak into their phones. They sit right next to you and talk into the stupid device for hours. You are made to listen to one half of a meaningless conversation while doing nothing. Can you not make a decision whether to take a call or not? If you must, can you not learn to keep your conversation short or tell the caller that you will get back to them later?
There has even been a lot of research done on the topic and why people find mobile phones annoying. You can see simple reasons in this work "Why are Mobile Phones Annoying?" published in the Journal of Behaviour and Information Technology. [Monk, et al]. Coming back to Thoreau's quote, we lived all these years without a cellphone. Life was normal and people found a payphone if they had something urgent to talk to someone outside of their homes or offices. Now suddenly with a phone in their hands, people cannot stop talking to each other over a cellphone. It definitely is a bane rather than a boon to have a cellphone.
[Monk, et. al] - Andrew Monk, Jenni Carroll, Sarah Parker, and Mark Blythe: "Why are Mobile Phones Annoying?" Behaviour and Information Technology, vol. 23, no. 1, 2004, pp. 33-41.