Although not an exact date, Nintendo of Europe announced earlier today that Pokémon Black 2 and White 2 will be released in Europe in Autumn 2012. The games are scheduled to be released in Japan in June 2012 so a simultaneous worldwide release isn’t the case apparently. Nothing has been announced about the American release yet but we expect it to be at about the same time, just like it happened with Black/White.
Update: American launch has also been confirmed for Autumn 2012 by Nintendo of America.