Courtesy of GhibliWorld, we see that Ponyo on a Cliff by the Sea has surpassed 10 Billion Yen, or roughly $100 million. This feat was achieved in only 31 days, and bodes very well for its continued success. While it's true that American blockbusters can surpass the $100M mark in a few days, that is due to a much larger population and thousands of screens. Japan has one third as many people, and only a tenth the number of movie screens.
All of which means Miyazaki has created another monster hit in his home country. Nobody, I'm sure, has any idea how far Ponyo will go. Within a few more weeks, it should share that same exclusive spotlight with Mononoke, Sen, and Howl as Japan's monster hits.