Hello,
After talking to Apple on the phone a few weeks ago (because my order was taking a long time to go from "Not yet shipped" to "Prepared for shipment"), the woman told me that "Prepared for shipment" essentially means this: the courrier has picked it up but has not yet updated its system.
It took a day to go from Prepared to Shipped, but if what she told me was true, then Prepared esentially means Shipped.
Loa