As reported Toyota will start to offer a smart key wristwatch for their Toyota Crown cars starting next week in Japan. Actually there is already a company in the Netherlands called Camco Systems that sells a wristwatch that opens cars remotely and also can start the engine and much more.

The Camco Systems Prostar has actually tons of features (see list below). It can even check the temperature inside the car.
The Camco Prostar car alarm watch sells for 239 Euros and can be installed apparently in any car. There is also a funny video showing the Camco System watch in action.
More details on the Camco Systems site. (Thanks Gregory S.)

Camco Car Key Wristwatch Features:
13 Features Programming
3000 Feet Operation Range
Arming Reminder
Automatic Central Lock
Automatic Re-Arming
Button Lock Out
By-pass Output
Cabin Temperature Check
Called Trigger Reminder
Remote car locator
Anti-Scanning
Anti-Code Grabbing
Cold Weather Start
Driver paging
Dome Light Illumination
Door, hood, trunk sensing
Door Lock / Unlock
Door Status Warning
FM Technology
Hidden Alarm
Silent Arming
Last door arming
LED status indicator
Low Voltage Detector
Operation error report
Panic siren
Passive Arming
Power Trunk Release
Protected system valet
Proximity Door Unlock
Real Time Clock
Parking Timer
Remote Engine Start
Starter Disable
Status Memory
Systems Status Check
Time Programmable Engine Start
Turbo Timer
Valet Mode
Vibration of Remote Control
Quick service

source

I hope they make a pocket watch version