Even a small country's stock market could be controlled by a group of investment banks, BTC surely can be manipulated by them, and they have been doing this for many years in every possible market - future, commodity, mbs... But currently BTC is still too risky for them, I'm sure when BTC becomes mature enough, wallstreet boys will come one by one
There is a major flaw with this argument. Bitcoin is not as 'elastic' as other currencies.
They are limited in amount, instantly verifiable both in quantity and quality (gold's problem), not subject to counter-party risk (equity based and no need for a lender of last resort), function as a settlement currency (MPEx options and futures) and are, most importantly, censorship resistant (another of gold's problem).
All of these reasons make the traditional methods of market manipulation much less, if not completely, potent. Coupled with gold, because every currency needs a mirror, Bitcoin becomes a Kryptonite dagger aimed at the heart of the State (fiat money central bank monopolies).