DoCoMo had a wristphone "Wristomo" in 2003
DoCoMo used to sell a wristphone called "Wristomo" back in 2003. However, it was a PHS phone, not 3G. As far as I remember DoCoMo also had a 3G wristphone at the Nagano Olympics but never offered it for general sale.
We have pictures of Wristomo on our blog entry from a trade show in Tokyo back in 2003:
There never was another Wristomo phone, so I guess there wasn't much more than a niche demand. The problem I guess is that you really have to take the wristphone off your wrist to phone or send messages, or do mobile internet stuff like booking airline tickets, defeating the purpose to some extent. If the demand would have been huge and continuing - we would still have lots of Wristomos in Japan now.