It's coming from China[1]. A LOT OF MONEY from China. Like $millions. That's how all these early adopters in the USA are being able to cash out millions and the price is not crashing.
They seem to like the fact it separates them from the US dollar, as well as allowing Chinese residents more control over their money and a potential store of wealth comparable to gold (although far more volatile).
Of course I still believe this is unsustainable, but in a strange form this is a wealth rebalancing act on a global scale... the rich Chinese giving money to the US citizens.
As long as the rest of the world doesn't go bitcoin mad and let the Chinese cash out again, then if it ever pops there will be a lot of wealth lost in China.
[1]http://www.bloomberg.com/news/2013-11-27/bitcoin-surges-to-1...