What decided the price at which that trade was executed?
As I understand it:
The buy order was entered into the queue when Bitcoinica's buy hit 6.8010. It then became a market order, but wasn't executed immediately on Mt. Gox due to a number of delays in the system. Probably the spike down to 6.8010 and below was quite steep, so by the time the buy actually went through, the price on Gox had fallen further.
You got lucky in other words. It could have gone the other way, too, and the spike might have only just touched 6.8, leaving your order unexecuted
Someone correct me if I got this wrong...
i think it is a function of the 4 sec delay btwn when u hit the buy button and when it executed. you were lucky enough to have a spike down during that 4 sec i would imagine.