New 21.5-inch Retina 4K models expected.
iMac Shipping Estimates Improve to 'Within 24 Hours' in Apple's North American Online Stores
Apple's online stores in other countries still show some lag in availability, with Australia seeing quotes of 3-5 business days for all models while Europe is generally seeing 5-7 business days for 21.5-inch models and 1-2 weeks for 27-inch models.
Even in the countries such as the United States and Canada where stock configurations are being quoted nearly immediate availability, build-to-order configurations are still seeing rather lengthy build times with quotes of 7-10 business days. The disparity suggests that Apple has been prioritizing the building of stock configurations in an effort to fill distribution channels, with custom orders holding lower priority.
Simple build-to-order customizations such as the addition of RAM or a different hard drive typically only add a few days to shipping estimates, so the current quoted timeframes should drop as availability continues to improve. Likewise, Apple's international online stores should also see their availability improve as increased stock filters throughout the company's distribution channels.