Cell phones, most definitely. Alas, I live in Finland, the promised land of cell phones (I'd like to think everyone knows Nokia, but whatever), where almost everyone who has turned 10 has a cell phone. My cousin's son got one when he turned 7.
One could think that as everyone has their own people would know how to behave with those things. But they don't. Trains, buses, stores, where ever you go, people have cell phones in their hands. They're texting, blabbering, listening to music and so on - and thus they pay no attention to their surroundings. People bump into each other. It's surprising how little people actually get run over by cars and buses while they only see or hear their cell phones.
The worst part is the people who feel the need to go through their parties, sex life or relationship problems via cell phone in a bus or train. I can't describe how much it pisses me off to hear someone blabber in a train about how her boyfriend cheated on him or vice versa.
This gets worse if I find out I'm already listening to music on max volume and can't get it louder. Thus I'm forced to listen to this person talk about his sex life which I couldn't care less of.

Of course, mp3 players or iPods have a bad influence also. People don't hear and if they talk, they talk way too loud. Makes me want to scream "Get the fucking things out of your ears if you're actually going to communicate with the world around you!"