There was a pre-open beta event that commenced over this past weekend. It consisted of two days of limited hours of play to test the servers to see if they were ready for the real open beta. Anyway, I got an e-mail from Sega this morning saying this (translated from JP→EN, of course) :
There was another, very long, segment of text under that that talked about the statistics and whatnot of the pre-open beta. Apparently, it had 41,500 participants online at the same time on the 16th.
If you're interested in playing this game, the open beta starts this Thursday. You can sign up at the official site, which you can find here : http://pso2.jp/players
If you don't want to wait, you can download the game client here : http://pso2.jp/players/download/