Text messaging is a way for unaccompanied non-smokers to look busy in bars.(I guess I probably should have Twittered it, but I'm still not very much of a Twitter-er.)
Put another way: Having a cellphone gives you even more button-twiddling ways to avoid eye contact with other people in public places.
After all, looking down at your watch is good for dodging the gaze of an oncoming stranger on the sidewalk, but you can only stare at your wrist for so long.
Add your headset of choice and you can be completely wrapped up in your own little world.