Prolific idea-person Chei Wei Wang has developed the "haptic clock," an application that forces your cell phone to vibrate periodically so as to tell you the time. Long and short vibrations indicate hours and minutes respectively. Hours are given on a 12-hour scale (six o'clock am/pm is six long vibrations) while each short beat indicates a five-minute interval (so three short beats means quarter after the hour). You can set it to communicate the time as often as you wish. In addition to being both useful and clever, the application absolutely pummels your battery (apparently vibrating takes a lot of energy).
What's interesting about Chei's approach is that it's using the phone's vibrator to signal something besides a ring. It actually tries to get data across--discreetly--using non-visual, non-audible means. This is not the first such attempt. There's been at least one shot at a "haptic vest" that (I believe this was how it worked--can't find a link) spelled letters and numbers on your back using an array of tiny vibrators. I doubt it ever made its way into production, but the idea is intriguing.
Pretend for a moment that the concept actually worked. If your back is an output device, interesting possibilities arise. First, you can receive text messages without taking out your phone--useful if you're walking down the street or driving a car. A couple of buttons on your Bluetooth wristwatch would control receipt, replay and acknowledgement.
Second, you could type text messages without looking at the screen. You'd always be sure which key you'd pressed, and a couple of extra control keys on the side of the phone would let you backspace or replay your message so far. Now you can compose safely while walking down the street or (not so safely, really--don't do it) in your car.
Games are another possibility. If you could put falling Tetris blocks on your back, you could play the game even as you made uninterrupted eye contact with the terminally boring speaker at the front of the room. The only giveaway would be your occasional out-of-context curses when the phone beat you.
Morse code might even make a comeback if haptic interfaces catch on. Jay Leno (talk show host in the US) once featured a face-off between two veteran Morse code operators and a pair of uber-texting teenagers. The Morse code operators won handily, a near-terminal blow to the kids' fragile Generation-Y egos. (The US Navy finally dumped Morse code a few years ago, but "chat" rooms on the net allow die-hard users to get together and bleep at one another in all seriousness. The irony of a morse code key--150-year-old technology--with a USB connector dangling from it is almost too beautiful to bear.)
In any case. I hope that Chei Wei Wang's little application heralds a more mainstream interest in safe and discreet communication and that you'll be open-minded about what I suspect feels a lot like mice nibbling on your back. But maybe you won't: Just as texting had to wait for a new generation before it really caught on, maybe haptic interfaces will have to wait til yet another generation (Generation-Z?) comes of age.