Pre-orders and previews begin April 10, launches April 24.
Apple Begins Preparing First 27-Inch iMac Orders for Shipment [Update: Shipping]
Customers who have received updated order statuses are reporting that they are still seeing their originally quoted delivery times, which are typically in the range of December 21-31, but it seems that their orders may arrive slightly ahead of that window if they ship out in a 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. A number of those seeing their orders being readied for delivery have also included built-to-order configurations, indicating that custom orders will be arriving essentially alongside stock models.
Update: Apple has now begun shipping these early orders, with many customers seeing delivery dates of December 18 while a few are seeing dates as early as December 13. We're also hearing from customers outside of the United States that their orders are similarly progressing toward shipment.