I read an article a while back about our ability to focus. Say you're studying something and you stop to answer your cell phone. In addition to the time it takes you to finish your phone call, it takes an additional 15 minutes for you to regain the concentration that you had before you decided to check your cell phone. So, if you spend 3 hours at the library, but stop to check your phone 4 times, then an hour of that study time is spent trying to regain focus and concentration.
I have this buddy that's addicted to his cell phone. Looking back on the last few times we've hung out, I can't recall enjoying the experience all that much. What I remember most is how often he kept checking his cell phone. Clearly his attention was on whomever was texting him at the moment. If you're so interested in who's texting you, then go hang out with them. I've made a decision to spend my time here with you. So, engage in the experience. Let's chat about the music, talk about the food, dish about chicks, whatever. I'm pretty sure this is what Eckhart Tolle talks about in The Power of Now. Life is short and precious. Why would you do anything half-assed? This is the thing I hate about cell phones: they have the potential to steal away significance and meaning from the simple joys of living. Technology has its place, but I don't want to live a life at a distracted 65%, and I don't want to hang out with people that are just about half way present. In college I had a professor that would take the first five minutes of every class to get everyone to quiet down, meditate, and "arrive" at class, to rid themselves of the concerns outside that particular classroom. I can't tell you how incredibly useful this was. I don't do this enough, but I think I'm going to start doing it more often.
Attention is an incredibly valuable commodity. Use it wisely on meaningful experiences with people that appreciate your presence.