these are his first posts yes?
http://sourceforge.net/mailarchive/forum.php?forum_name=bitcoin-list&max_rows=25&style=nested&viewmonth=200901From reading them he doesn't strike me as Japanese born, the english is way too smooth unless perhaps he had one western parent. I know a lot of Japanese including plenty that have spent decades living and working in the west, all of them give themselves away with a few paragraphs of spoken or written text.
What was his email back then? Satoshi@vi.. viwhat?
also this was news to me but what do I know
"For now, you can just multiply the total blocks by 50. The Bitcoin network has been running for almost a year now. The design and coding started in 2007."
the way he writes that sounds very much how you'd write it if you were heading a group, one that only went public one to two years after getting the basics working.
Also given that he spells "realize" and so on with a Z, his style is firmly american english not british english.
As a resident of Japan who speaks to non native English professionals in many fields everyday,
that whomever wrote the messages in the link above is either a native English speaker or has spend the vast majority of their life in an English speaking country. From the above link I didn't see any clear indication as to which country though.