The company released on the holiday weekend a Support Note about the incompatibility after several news reports on the issue. It says that the new iMacs, called "Late 2012" are not "VESA mount compatible at this time."
The mounting interface standard from the Video Electronics Standards Association enables swivel-arm mounts from a wall or desk. Apple offers a $39 VESA Mount Adapter Kit for iMac and LED Cinema or Apple Thunderbolt Display, which supports 24- and 27-inch displays and iMacs. Or they did until now.
The wording from Apple suggests that a compatible adapter is forthcoming. However, from what I understand, Apple currently doesn't offer a VESA-compliant solution for smaller screen-sizes (it may never have).
However, third-party companies such as ErgoMart and iMacMount.com offer adapter kits for these smaller models. iMacMount offers a $79.95 kit for 17-, 20- and 21.5-inch models. The company's site says its current iMac Mount VESA Mount Adapter Kit supports the now problematic Late 2012 21.5-inch iMac.
In addition, a version for the 27-inch iMac will ship in January, the company said.