As an update, I called to get the scoop on this and here is what I was told:
It takes about 10 days for a car to be built form start to finish at this point in time. As a result, VINs are being assigned about 10 or so days before the car is due to be completed. No reason to worry unless you are less than 10 days prior to the last day of your window and have not yet been contacted.
FWIW, I hope that helps.
Cheers.