I think computing power would be a much bigger issue than bandwidth. You'd need to run many instances of the game for each user, and continuously kill and fork them.

And frankly, a $500 computer today can already display quite good graphics. Maybe by the time gigabit bandwidth is ubiquitous, computers will be usable enough that installing and running a game locally will be as seamless as clicking a link on a web page, so we won't need cloud gaming amymore.