Apparently, apple offered optional SSDs at the time of purchase, but the space and sata ports are available in all 2011 iMacs.  You will need some custom cables and a bracket (or double stick tape) to complete the install.  It is also a pain in the ass (requires a full Logic Board removal) to run the cable to the board.  Not recommended for novices, but its incredibly cool to have both the speed of SSD and the capacity of a mechanical drive, all inside the iMac.