I think they actually have two at the Ring Of Fire in Osaka. My wife and I just spent the past two years in Tokyo, never did make it down to see that aquarium though. As a diver I tend to want to wait to see one out in the wild though where it belongs, they are just to big and wonderful to be kept in captivity.
As a side note, down in Okinawa there are also some fishermen who keep one or two in a netted in lagoon and for $100 or something you can go diving with them. Again, not my cup of tea but its out there.
-Scott