Apple has started shipping its iMac with a built-in VESA Mount Adapter.
The mount is available with all configurations of the iMac and only adds £36 to the price of the iMac. Unformtuately this is a CTO option only, so the VESA mount for the 2012 iMac cannot be purchased separately should you already own a 2012 iMac.
The addition adds just £36 to the price of the iMac, bringing the price of the entry level 21.5in iMac from £1,099 to £1,135, the 21.5in 2.9GHz model is £1,285 (up from £1,249), the 27in 2.9GHz version is £1,534.99 (from £1,499) and the 27in 3.2GHz version is £1,735 (up from £1,699).
Apple sells a wall mount for £35 on its online store here, but notes that it is "Not compatible with iMac (late 2012)." It appears that the older wall mount wasn't compatible because the new thinner iMacs have a compound curve on the back and the VESA mount needed a flat surface to rest against. There is also an access plate for the RAM in the area where the mount would be attached and there is some suggestion that that could weaken the case.
VESA mounts are commonly used with televisions and for retail fixtures and in hotels and receptions.
A number of forum posters on MacRumors were upset that the iMac 2012 edition couldn't be wall mounted.