If you didn't already order your new 27-inch iMac, it looks like now you won't get to meet your big, thin computer until January. As first reported by Ars Technica, the product records on Apple's website now indicate that the 27-inch iMac is "available to ship" in January.
Now, if you pre-ordered your 27-inch iMac already, don't panic. Instead, check the shipping estimate you received; if it indicates a late December ship date, Apple intends to honor that promise. The delayed shipments should affect only new orders.
The smaller, 21-inch iMac started shipping at the end of November. Apple had announced near November's end that the 27-inch iMac would start shipping in December, and while that may have been the case, it looks like supplies are too constrained for new orders. If new 27-inch iMacs ordered today ship January 2, that means Apple's production lead time is at least 3.5 weeks at this point.