Theories on why mount incompatible and options
I too had been planning to arm mount my new 27" 2012 iMac after having my 2007 24" sitting on a dictionary for years and not being able to adjust it for various members of our family. Once I saw the fine print added to the VESA bracket's page on the Apple Store, I started looking for alternatives. One was as noted above at Ergomart, and another can be found at imacmount.com. Theoretically, either approach could be a solution for the 27", but the manufacturers will need to wait to get measurements of the 2012's leg. I emailed the guy who runs imacmount and he seemed interested in providing a model that would work once he has access to one to test out.
As to why the 2012's can't use the Apple bracket I have a few theories (and it may be a combination of the issues), which I am sure fellow forum users can debate. See the photo at AnandTech for reference:
http://www.anandtech.com/show/6402/up-close-with-the-new-27inch-imacs-user-serviceable-memory-panel
1. I think the back of the new iMacs have a compound curve on the back, so the VESA bracket would not have a flat surface for its top and bottom parts to rest against.
2. Having the access plate for the RAM on the back might weaken that area of the case and make it less suitable for the bracket to rest against.
3. The bracket seemed to bisect the vent on the previous generation of iMacs and maybe Apple decided they didn't want (or couldn't allow) airflow to be restricted. (Actually, on the older iMacs this may have been an intake, as those Macs still had a slit at the top for venting, right? Can any 2011 owners confirm this?)
4. The volume required for the release mechanism might be too large in this new thin form factor.
5. The thickness of the aluminum case might have been reduced in the 2012's, again reducing strength for alternate mounting. (Did you notice they weight 20 lbs. instead of 30 now?)
Let's hope they just will be coming out with a different kind of bracket, but I think the chances are as slim as the edge of one of these machines...
--JV