YM a device that royally screws you when it gets stolen?
Ahh but part of the deal would be the watch is locked/deactivated when it goes out of range of the iPhone.
You could also have alerts when one is left behind (unless of course you mean to do so at which point you would be able to turn said alerts off).
No more losing your phone. I assume a watch could also go into ultra-low power mode and only put out a GPS signal for easier tracking if it is stolen.
A big want of mine is biometric authentication - so if the watch is worn by someone other than the person who sets it up, it doesn't work......but I don't know how feasible that is at this point.