Constantly Connected

I was chatting with a friend of mine and we were discussing how connected most people are today. Between the cell phone, email or some web version of chat, most people are reachable virtually 24/7. For so long, people have strove for exactly that. The desire to be more connected at all times, so if you wanted to reach someone, you could. (The first cell phone call from the peak of Mt. Everest occurred earlier this year. I read that the reception was terrific because of clear views of towers in China. And I drop calls downtown!)
Anyway, the question asks itself, when does it become ok, to opt out or purposely make yourself unconnected. If someone knows you screen your calls, they always assume it's not for them, but when is it still polite to say, Yes I'm connected, but I'd rather be 'unavailable' right now. How will this question be answered for teachers? I know as a teacher, if a student sees that I'm online, and asks for help, I don't want to not help them, but is there an etiquette developing for this level of connectivity?
I always marvel how, if I'm standing at a counter at a store and the phone rings, the clerk will stop helping me, and work with the person on the phone. I feel a little like, "Hey, I came into the store, when did the phone trump actual human interaction?" I'm not really upset by it, I just question how far can it go. If I'm constantly connected and get emails or Instant Messages, does politeness require I answer those before having the face to face conversation?
Just some summer time thoughts . . .