Sure, a lot of us are excited about the upcoming US release of Ponyo (August 14th!), but some of us are saddened, too-- after all, who knows how long we'll have to wait for the next Ghibli film?
Good news-- not very long at all! Studio Ghibli's next film will be released in 2010, according to Nausicaa.net (translating the Studio Ghibli blog). Of course, we have no idea what that movie is, and of course it might not be Hayao Miyazaki's next film (though it could be attached to another big name, like Isao Takahata).
In fact, Wikipedia specifically cites a summer 2010 release for an Isao Takahata film called Kerou's Rebellion-- they cite Nausicaa.net for the information, but it no longer seems to be on the site, and everywhere else I look just links back to the same Wikipedia article. So I'm going to just call that one a “maybe.”
What kind of movie do you want it to be?