According to Japanese site Quiter, Kingdom Hearts II has indeed been confirmed to be in development by Square. No details are available so far, but RPGFan will keep you posted on all future developments.
Square officials have already hinted at the possibility of a sequel to their 2002 blockbuster title whose two versions have sold more than 1.2 million copies in Japan alone. Back then however, Square president and CEO Yoichi Wada noted, that a decision regarding a possible sequel had to be made by Disney, whereas his company was definitely interested in making a sequel.
In further Kingdom Hearts-related news, producer Shinji Hashimoto and director and character designer Tetsuya Nomura will receive the IGDA's Visual Art Award at the Game Developers Conference in San Jose, California.