Some people who ordered the Retina iMac in the past few weeks have been receiving emails from Apple, claiming an "unexpected delay" in shipping. In one
example, a buyer says they placed an online order on October 31, and were initially quoted a delivery window between November 14 and the 19th. A delay notice has shifted that window to between the 28th and December 2.
As usual, Apple hasn't provided an explanation for the delay. Recently, however, listed US shipping times were stretched from seven to 10 days to
two to three weeks, which suggests that Apple is simply having a difficult time manufacturing the new iMac in quantity. In that sense, the most likely source of delay is the 5K screen, since Retina-level displays can be difficult to manufacture.