As someone who holds Bitcoin this is the one thing that has concerned me the most. Mt. Gox has seemed incredible shady. What if they have been manipulating the market all along?
There's been a lot of speculation that they have been manipulating it, all the way since the rise from $10-something to $250+ last year. If you watched the public trading, it seemed like Mt. Gox was intentionally blocking 'sell' orders, refusing to sell bitcoins at market price and instead waiting until someone placed an order for slightly more money.
I forget what the other 'evidences' were but it seemed like they were trying very hard to drive the price up. And they would frequently shut down altogether if the price started to dive.
Most people who were paying attention considered Mt. Gox toxic and avoided them as much as possible since the $250 peak-then-crash of last year. It wasn't just the can't-really-prove-it suspicions, but also things like a trading engine lag of 4 hours.
Recently, a lot of the price fluctuation seems attributable to what Mt. Gox has been doing. When the price at Mt. Gox fell to $100, imagine how easy it would be for Mt. Gox admins to buy btc from their customers and then sell at bitstamp/coinbase for a giant return. Rinse and repeat. Only Mt. Gox should have been affected, but all of bitcoin tanked, suggesting that maybe Mt. Gox was actually doing a lot of selling.
None of this is proof. I have no proof more solid than what I stated. But I'm fairly convinced that Mt. Gox has intentionally and successfully tampered with the price of Bitcoin for the past year or more.
