If do not know, the latest iMac models, specifically the 27 inch ones, had a really annoying bug – their screens start to flicker. Apple acknowledged the problem and roll out a software update to fix the issue some weeks ago, but judging from the forums the update didn’t fix anything. So Apple set a 3 week delay for the shipment of this model iMac and this week roll out another software patch. I will search for more info and update the iMac guide.
You can see more info about the patch here