According to number of MacRumors forum members, customers begin to see changes in their order statuses to “Preparing for Shipment” and their credit cards charged for their early orders of the new 27-inch iMac. The new status indicates that the orders can no longer be changed or canceled and is the final stage before the machines are passed off to delivery companies.
Customers are still watching the originally quoted delivery times which is in the range of December 21-31. But it looks like their orders may arrive slightly earlier if they ship out in timely fashion. So far it appears that only U.S. customers have reported seeing their orders being prepared for shipment, but there are still only a few data points available.