Apple has begun offering iMacs with an optional VESA mount adapter. The new BTO option was first noticed by Macworld UK. Users who buy an iMac on Apple's online store can order the iMac with the VESA mount adapter instead of the standard foot stand for an additional US$40.
The pre-installed adapters cannot be swapped out for the iMac's standard foot stand later. Apple is currently not selling the VESA mount adapters individually for use with previously purchased slimline iMacs. Users of iMacs with VESA mount adapters will also need to purchase a mounting arm separately, as the iMac does not ship with one. From the description on Apple's website:
The iMac with Built-in VESA Mount Adapter is ready to pair with your favorite VESA-compatible wall mount, desk mount, or articulating arm (sold separately). This iMac doesn't include a stand, so a mount is required. If you don't already have a mount, you can purchase one when you configure your iMac.
Apple currently lists a seven- to 10-day ship time for iMacs with VESA mount adapters.