Microsoft is taking the George Lucas approach to the Halo franchise, according to an article at the Los Angeles Times. By exercising more creative control over the franchise as it continues to expand in different areas beyond the game, they want to build a larger narrative. One of those narratives is to follow what other companies has done rather well by providing short form stories that will appeal to a segment of the existing market, namely anime fans. After the success of Animatrix and Batman: Gotham Knight, Microsoft is working with a few different Japanese anime studios to creative a similar product for the Halo franchise that's due out for release on DVD and Blu-ray as well as digital release (presumably through Xbox Live) come 2010.
The known studios at this point are Production I.G., Studios 4*C, Bones, Casio Entertainment, and Toei Animation with Shinji Aramaki of Appleseed fame and anime legend Mamoru Oshii providing the guidance for the project overall in Japan. The main visuals and storytelling decisions are coming from Microsoft in order to make sure everything ties together well into the growing universe that they want to promote and explore.