I believe that central banks can buy/sell bitcoins in volume to manipulate the price, as they do with precious metals certificates.
They're already doing it. Coinbase is a Goldman Sachs operation. The Big Dogs are quietly trying to accumulate as many bitcoins as they can without driving the price up. At the same time, banks are making it difficult for smaller bitcoin businesses like Mtgox and others to use the banking system.
I have great faith in Bitcoin's concept and the crypto behind it. I have no faith in the majority of Bitcoin's users and developers to have an inkling about who and what they're up against.