Casio announced (but hasn't yet released) a new watch that integrates with a phone via Bluetooth:
"Your watch can vibrate when you get a call, text or e-mail and show the person’s name, even if your phone is in your purse or briefcase. The G-Shock can help you find a phone lost in your sofa by making it chirp loudly. It can also beep at you if you leave the phone behind in, say, a restaurant.
This arrangement makes a huge amount of sense. How many times have you complained that you missed a call because you didn’t feel the phone’s feeble vibrating in your pocket? Now you won’t be able to miss it; the vibration takes place right on your wrist. (You tap twice on the watch’s face to send the call to voice mail.)
You know how cellphones set themselves automatically as you travel from time zone to time zone, courtesy of the cellular network? Well, now your watch can set itself, too, since it’s talking wirelessly with your phone."
Note that (a) it's not available yet, and (b) phones that support the forthcoming watch are ... thin on the ground. Eg., the iPhone 4S supports it in theory, but not yet in practice.
Read more: A New G-Shock Watch Awaits a New Bluetooth