This has been discussed many times. I’m not finding an “official” statement, but you can put a number of things together that strongly support that shipping = initial acceptance letter.
For instance, GF promised that Pros would start shipping by the end of June. To the Forums knowledge there was only ONE email that went out before the end of June (June 30th), in this thread:
At the same time, GF declared successful delivery of the first Pro in June. The Pro customer just got his Pro right at the promised 6 week time period (I think to the very day).