This type of attack has always been my fear with Bitcoin. Since the majority of Bitcoin miners are in China and the Chinese government is Communist, at any time, the government "owns" the miners that are running in it's country.

So ... if over 50% of the miners reside in China, then effectively, a single entity (the Chinese Government) has control of over 50% of the network.

I know, I know, it's a conspiracy theory ... but it still occasionally worries me.