Defiance is Trion Worlds action-shooter MMO that will tie-in with the SyFy series of the same name. What happens in the game's San Francisco will be referenced in the TV series' location of St. Louis.
Of course, the MMO is much more than just a crosspromotional tie-in. In the first dev diary, much of the game's mechanics are discussed by the developers. Check it out in the video below, as well as the E3 2011 preview.