Toei Animation's new anime movie "Niji iro Hotaru ~Eien no Natsuyasumi~ (translation: Rainbow-colored Firefly ~Never Ending Summer Vacation~)" was announced at Tokyo International Anime Fair 2009. The release data was not given.

Picture of the poster at TAF2009

"Niji iro no Hotaru" is based on Kawaguchi Masayuki's novel.
Cover of the novel
Synopsis
Yuuta was 12 year old boy, who had lost his father in the traffic accident one year ago. In the summer vacation, he visited a deserted dam deep in the mountains, where he had a good time with his father before. Suddenly a thunder storm occurred and he slipped on the ground. He lost consciousness and woke up to find a girl and an unfamiliar village. He time-traveled 30 years and reached a village, which sank at the bottom of the dam. This is Yuuta's precious memory of "another" summer vacation.

Source: Japanimate.com