I definitely think there's an introvert/extrovert dimension to it, just as there is to who walks down the hallway looking for people to go to lunch with, and even who notices when someone walks down the hall. People vary widely in their ability to screen that kind of chaff as well.

But that's not all of it. I grew up with several siblings and I have four children. I can work or read without difficulty completely surrounded by noise, people and chaos. At the same time, if in the middle of that chaos someone says "Mom", or mentions a rare topic I'm interested in (say, Amtrak), I'll hear it and surface from my book/task to see what's going on. There seems to be an entire subculture growing up which has the same ability to pick what they want out of the electronic noise. I'm definitely not there yet, but I've also definitely observed it.