So after all those MBAs showed up as Prepared for Shipping and then went back to Not Yet Shipped, what do you suppose we are waiting for? Are they just going to suddenly unleash a huge shipment?
I think they initially went to "Prepared for Shipping" because of whatever automatic mechanism Apple has in the background to manage their online ordering system. Apple likely has an internal launch date which is partially based on the delivery of samples to professional reviewers and deployment of the ad campaign.
If 10.5.2 was the reason for the 'holdback,' there is no way "incomplete" 10.5.1 units would have found their way into the hands of people like Mossberg and Ed Baig.