Since miners only produce about 7,200BTC a day, do they really have any power to demand higher or lower prices on a market with volumes of 40,000 to 100,000BTC a day?
The market volume is not the right figure to look at here, because it includes short trading. If I had to make a guess, most of the daily volume is generated by people pumping in some money into the economy by doing short trading). Let's assume there's 100 people on Mt. Gox with 1000 USD on their account doing speculative short trading. Let's also assume each one of them uses all of their USD for the short trading. That's ~66 BTC per trade. Now, person A buys 66 BTC from person B, person B buys 66 BTC from person C and person C buys 66 BTC from person A. We have now a 66*3 = 198 BTC trade volume, but there's no actual USD added to the economy.
What I assume, is that some of the miners are selling their BTC, and that's where the extra USD comes in the economy. Otherwise, it's mostly speculating and short trading, hence the large volumes; the same USD are circulating and the speculators are playing a zero sum game with the exchange rate
(Or actually, a minus-sum game, since the only one who wins for sure in that game is Mg. Gox...)