In January, shipping times for the 21.5-inch iMac slipped to 2–3 weeks, and the 27-inch iMac has been in short supply since its November release. The supply shortage has been attributed to issues with display production resulting from a new lamination process.
During Apple's first quarter financial call, Tim Cook noted that iMac supply constraints in the fourth quarter had impacted sales results. He expected supplies to increase during Q1 2013, which was confirmed in late January with a report that assembly rates for the iMac had been improved.
Apple's redesigned iMac is considerably thinner than its predecessor, with 40 percent less volume and a 5mm edge. The newly implemented lamination process provides improved optical quality and an anti-reflective coating.
Source:
http://www.macrumors.com/2013/03/02/shipping-estimates-for-21-5-inch-and-27-inch-imac-drop-to-1-3-days-in-us-and-canada/