Apple's sleek new iMac is hard to produce. Supply is constrained and the 21.5-inch model is facing large delays and the 27-inch hasn't launched yet. Apple had announced on November 30 that the new 27-inch version of the iMac would be shipping within 3 to 4 weeks, though that has now been changed.
The new shipping estimate places the Available to Ship time as January. No date, no week estimates, nothing solid to go on, other than the units will be available to ship sometime in January. This could mean that the systems will be shipping as late as the end of the month, especially if Apple is facing supply constraints and other delays.