According to an official press release made public earlier today, the CEO of NCsoft Japan and Global Marketing Director of NCsoft Korea, Mr Kim Taek Hun (below), confirmed that Blade & Soul Japan is currently in the works.
This comes after the China (link) and Taiwan (link) servers were confirmed earlier this year.
In the statement made, Mr Kim talked about how he has had successes leading Lineage II in Japan since the game started services in 2003, and that he is currently traveling between Japan, Taiwan and Korea to prepare the 3 Blade & Soul services for launch.
Although there is nothing said about a simultaneous launch, it does seem possible currently.