The New York Times reports that CBS will broadcast footage from the World Series of Video Games on 7/29/07 at noon. There are, of course, other cable channels that cover games, but they're decidedly niche. And as frequent game reporter Seth Schiesel notes, it'll be a first to have some guy channel surfing for a golf match suddenly come upon Guitar Hero II and WoW on the tube. With the shrinking impact of any one TV outlet, I don't know if this qualifies as a watershed moment, but it's still a major network covering gaming (and an MMO, to boot), so it sounds like at least a minor milestone.
I'll be watching just to hear the announcers explain to America why a mage would iceblock (because ice mages are irresponsible aggro-pullers on boss fights).