If you want to combine a Bluetooth headset and wristwatch in one unit, Chronotech has the answer in its Bluevoice device. When a call is received, the detachable face snaps right off and fits snugly into your ear. It comes in analog or digital models, with prices starting at $119.95. Product page here.
When the call ends the hands free unit can be re-attached onto the strap, providing maximum comfort as you do not need to wear the main unit on your ear at all times