Cellphone: Human interaction resistance +20?

I just took the tube ( tube = metro, subway, sossesnöret, grävlingen, bunneltranan) home from a nice date just now and noticed a behaviour that I've got myself. That does not make it any less weird. Cellphones are used to make the user look occupied. I mean: sometimes you probably play a really cool game, but I suspect that most of the time people just click back and forth between menus/flip between SMS:es/do nothing at all really. I've seen people do it, I've done it myself. It's kind of the same behavior that makes me uncomfortable sitting in a café alone unless I've got a book or magazine to read.
It's kind of a new version of staring with huge ( fake ) interest at the commercials. Both the new and the old versions are tools to avoid eye contact with your fellow travelers... Why are we so desperate not to interact?
The last time someone spoke to you on the tube/bus: was your initial reaction "Oh no"? I mean that split millisecond before your brain sorted out that the guy just asked the time... And as you started fishing out your cellphone to see what time it is: did the thought "Is my cell too cool, is he just checking if I'm worth sticking up later?" cross your mind?

Why haven't I ever asked the lad/lass across from me: "Hey, I'm meeting a friend for coffee in 10 minutes, do you know a cozy café in the south part of town?"