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In korea it seems to be Seoul (the captial) and Busan (another big city) (i see a lot of korean cube forum posts from people from Busan)
And of course Changwon, only because cubenjoy is there and ships from there.
But korea is such a tiny country anyway, just a bit bigger than Kentucky area wise.
It does have about 50 million people though.
NOTE: All mentions of Korea, I mean South Korea. I highly doubt cubes even exist in North Korea.
CubingUSA's not going to tell you this, but at given time during the school year there are over 1000 people on the Berkeley campus who can solve... now if you're talking about solving fast (sub-20), that might be a different (at least an order of magnitude fewer).