It's getting there but it still has a long way to go. Casual games have pretty much hit mainstream, so stuff like Angry Birds, Temple Run, Call of Duty etc. are in the mainstream minds.

Traditional style gaming however, is still very much a niche topic that people who do play are often embarrassed about. My manager for instance is a huge Diablo fan and used to play Age of Empires II, but he doesn't like to talk about it too publicly. He's nearly 10 yrs older than me though, I suspect younger people/generations willo be able to talk about games without so much embarrassment.

Looking at the originality of the most succesfull games i'd say yeah pretty much mainstream and sadly so.
From the aspect of gamers and their social surrounding, not entirely, maybe among youngsters but adults not so much.

If you consider Angry Birds a video game, then yes definitely

They're never going to match movies and music, simply because video games demand more involvement, even at their simplest. People are going to want passivity from their entertainment more than active engagement, perpetually.

That said, they have certainly gained mainstream penetration.

Yes they are,mostly thanks to Sony