Nikkei.Net (subscription) reports that Toyota will introduce a Citizen Wristwatch with a smart key inside to lock and unlock cars automatically.

For new Toyota Crown car owners the fumbling for the car key has an end. The Toyota wristwatch (developed together with Citizen) transmits via a radio signal a code to unlock the wearers car.
There is also a button on the watch to manually lock or unlock the car from a distance. The Toyota wristwatch will be available starting next week in Japan, where else. Toyota released now some details about the Toyota watch on their site (Japanese).

See also the RFID reader watch.