It was announced at Comic-Con. The anime is going to be called Halo Legends and this will be done like the Animatrix and Batman: Gotham Knight movies. Looks cool!
The Trailer
latimes article
I don't play video games, but the Halo series seems to be kind of an action version derived from a lot of literary SF space operas.
It seems like a lot of people who watched it arm being overly critical because it's only a trailer that announces that the project exists, and who produces it. It doesn't look like a trailer meant for mass consumption.
RAMA