The max waiting time for the late 2009 27 iMacs were at most 4 weeks.
So us waiting 2-3 weeks for mid to late December delivery means even with a back ordered. The most anyone will wait for new 2012 iMac would be 4-5 weeks max.
Apple knows the game. Any time window in the tech world more than 5 weeks consumers lose focus and may cancel their order.
It doesn't take more than 4-5 weeks in mass tech production for anything with Apple's supply chain.