iFixit is done with the 21.5 inch teardown. Any thoughts on if users can add an SSD in there with an aftermarket drive? Looks like there's bolt holes to hold something, but I'm not that familiar with what cables would be needed or could be split.
http://www.ifixit.com/Teardown/iMac-Intel-21-5-Inch-EMC-2428-Teardown/5485/1
http://www.ifixit.com/Teardown/iMac-Intel-21-5-Inch-EMC-2428-Teardown/5485/1