Really? Judging by bitcoincharts.com mtgoxUSD 30d volume of 844k to EUR's 96k, I would have guessed it was US based.
Really? Judging by Google Maps Tokyo's coordinates are 35° 41′22.22″N 139°41′30.12″E, I would have guessed it was Japan based.
I guess that depends on whether "center of gravity" refers to where Bitcoin gravitates to to get converted into fiat, or what nationality gravitates toward Bitcoin.