A strong, technical party, say a HFT trader, could manipulate the market probably very easily. Bitcoin is still rather small. I would wonder, what would happen, if a financial institute starts buying and mining bitcoins and hide them after that. Thus making the amounts of bitcoins smaller and smaller and then at a certain moment, dump them quickly through various (unconnected) channels.

Is there some way to check the amount of circulating bitcoins? That would be interesting.