In short: no, nothing is known.
There was a bit of hubbub a few weeks ago when someone at Game Freak or Nintendo said that Black & White were "the end of the story" which got interpreted as there wouldn't be a 3rd game. But it seems like they were wrong or mistranslated.
There is almost certainly going to be a 3rd game. If it follows the patterns of the last generation then it would be released in Japan in September 2012, and elsewhere in Spring 2013. Announcements would come very late this year or early next year.