The film is The Tale Of The Bamboo Cutter (Taketori Monogatari), an adaptation of a popular folk tale about a princess discovered as a baby growing inside the stalk of a glowing piece of bamboo.
Word of the project has been circulating for a while now but Studio Ghibli have just formally announced the project along with word that Takahata has co-written the script with Sakaguchi Riko while Ikibe Shinichiro is composing the music.