This is an old idea in the world of telly-and-games, and a really, really hard one to pull off: I'm kind of both amazed and not surprised that it's taken until 2012 to get it to happen. Enormous congrats to Syfy!
Defiance is both a TV show and a videogame with plot touchpoints that affect both:
With Battlestar Galactica and Lost now distant memories, there's a gaping hole in the "epic sci-fi" category on television. Enter Syfy's Defiance.
The television series is being developed in conjunction with a massively multiplayer online game, and the two will be launched simultaneously. They'll be connected by the same overall storyline, to the point that events from the show will play a part in the game, and vice versa.
Defiance is set on a future Earth ravaged by war, and will feature human and alien cultures attempting to rebuild civilization in a post-terraformed Earth.
The only crossover I've experienced to date (I didn't play the Battlestar Galactica BigPoint game) was an early moment in Star Wars Galaxies where you get to find the huge dead dinosaur-bone-set that R2 & Threepio meet after their Tatooine landing. I stood there for ages, taking pictures like a tourist ;-)
It was magic.