This is the issue I struggle with ATM. Realizing that every transaction that goes through Bitpay results in an immediate sell order at whatever the current best bid price is on Mt. Gox will actually help drive the valuation lower. We need more speculators and big investors putting the money up to counterbalance this with an upward trend.
The blindness is bizarre. How can you ignore the fact that the BTC used in the transaction also has to be bought with fiat? In order for the BTC to be sold on Mt. Gox by the merchant, it first has to be bought by the customer (on Mt. Gox perhaps). The two currency transactions balance each other perfectly.