If you've ever been around more than one teen-type-human, you've probably seen something like this. They are all in the same place at the same time, but they're all doing something on their own mobile phone. They might be texting each other or they might be texting about each other. No one really knows.
Teens do what they do. Trying to force rules of etiquette on them is tantamount to reversing the tides. It may happen, but you have no power over it. However, the habits of teens can bleed over into the real world and infect the rest of us with poor habits.
How does it make you feel if you're having a conversation with someone and they stop in the middle to look at their phone? What if they interrupt your conversation to have a conversation with someone, either through a call or text message? I know that I feel less important than what's going on through the phone.
In some cases this is OK. For example, if several of us happen to meet in a coffee shop and we drift into a conversation, then it's not terribly rude to drift out of a conversation to attend to a phone message. If, however, you invited someone out for a meal or coffee and they proceed to ignore you in favor of the super-computer in their pocket, that, in my opinion, is rude.
To me, the standard should be presence. Ask yourself the question: How present do I need to be in this moment? If you could also be reading a book or flipping through a magazine, you don't need to be very present. But, if it would be rude for you to pull out a magazine and start flipping pages, then it would be rude to pull out your smart phone.
Another issue is the ubiquity of Google. Let me tell you, I love Google, but it can interrupt conversations terribly. If we don't remember the name of the actor in the movie as we're talking, it's just too easy to grab a phone and Google the answer. But that's the same as pulling out a reference book and looking through it. I'm not usually talking to people so I can get the most accurate information, but rather so that I can make a relational connection with them. Google will never be able to help me with that.
How do you deal with the ability to be online when you're face-to-face?