Magic: The Gathering - Battlegrounds.
It is mentioned in a past Magic Arcana.
It is an action/strategy game for your Xbox or PC set in the world of Magic: The Gathering.
It is playable online via Xbox Live or over the Internet.
It was developed in close association with Magic R&D (Elaine Chase worked on the game's mechanics and Brady Dommermuth worked on issues of setting and flavor).
It releases on November 18th.