Yeah, I've thought this for a while now. It's like it used to be you either were available to chat or you weren't, but now it's like everyone is half-available at all times. I agree with other commenters that the fact that you had to be sitting at a desktop was also a factor. Finishing what you were doing and logging into AIM (or taking down your away message) was the equivalent of opening your door, going into the lounge/breakroom, etc. --- a signal of openness to interaction.