As a heavy used of my android-powered smartphone, I know the value and pervasiveness of this constantly connected piece of computing power. its a reflex to pull out my phone at any pause in the daily goings on of my life. To check the time, to check for new messages either from email or sms or twitter.
Having a high resolution color touch screen in my pocket is an amazing gateway to a huge amount of information. It goes with me wherever i go and its always there ready to provide information or just entertain. New Android phones have a 1ghz processor, 256MB of ram, and gigabytes of storage. There is a barrier that I believe is about to be punched through and that is the headmounted display. Coupled with augmented reality, a head mounted display will be a quantum leap for human-machine interaction.
A new message would make itself known by altering what you see in some way. Eyeglasses would be the interface - easy for people who already wear glasses. I guess that number is smaller than it used to be given contact lenses. They would be transparent except when the computer needs to display something else in a part of the screen. this is not augmented reality as the glasses would also need a camera so the computer can see what you're seeing. With high enough resolution, part of the view could be obscured to display information.
A new level of ambient information will be integrated into one's daily experince. It will create new mental conditions and addictions like never before. It will be terribly useful but like the automobile, we have to be aware of what we're giving up at the same time.