Originally Posted by
railroadtycoon
You're still a citizen until you go through the US government process of renouncing citizenship, to my knowledge for Americans, the Japanese Government doesn't ask for any proof of that you went through that process.
This is correct--though technically, if you naturalize in Japan and then fail to de-naturalize in the US, you have committed fraud in your Japanese naturalization and could have your Japanese citizenship taken away.
My main point is that this is not the procedure for all nationalities. Britons have to de-naturalize in Britain BEFORE their Japanese naturalization is completed. I don't know what the situation is like for Canadians.
There's a lot of information and anecdotal evidence here:
http://www.turning-japanese.info/