On the Studio Ghibli blog, there is some information about Miyazaki’s next film:

At Monday’s press conference for Kokuriko-Zaka Kara, Hayao Miyazaki discussed the movie’s theme song, Sayonara no Natsu, and reveals details into the pre-production of his next feature film.  If Ghibli is still following their “Five-Year Plan,” then Miyazaki-san’s following movie will be in production for two years, and be released in 2013.