I saw a chart earlier today with monthly pre-order numbers. They went something like this:
July: 120,000
August: 244,000
September: 63,000
October: 229,000
November: 246,000
So, roughly 50% of the pre-orders were July-September and the other half October-December. Those numbers should allow an ambitious math geek to get a better chart for suspected start date.
Note: I didn't verify the chart as accurate so take it with a grain of salt.