I don't think bitcoin is being thrown at people's faces just yet, at least not in the United States. I'm not sure the "London underground" qualifies as mainstream. In any case, bitcoin carries some risk just as all emerging technologies do. By diversifying, the impact of its volatility will be minimized.
5 million people that the tube carries daily seeing those ads and kiosks might be called mainstream.
They are popping everywhere because they charge a lot in fees. I've seen prices from 5% to 10% percent.
Almost no running fees, lots of morons eager to get rich, you just have to sell 10k worth of bitcoin monthly to take some nice profit home.
And that is less than 300$ per day, easy to achieve in a city with high wages and millions of people.
But I doubt it's a sign for a bubble waiting to pop.
Slight downfall maybe, back to 250... no way.