iFixit, who are well known for providing tear downs of all the latest Apple products, have released a kit and guides for installing a second hard drive in your 21.5 or 27 inch mid 2011 iMac. This option is down to the discovery of two unused mounting points in recent iMacs while iFixit were performing their teardown.
The kit can be purchased here for $69.95, while the guides can be found here for the 21.5 inch iMac and here for the 27 inch model. The kit includes a Custom SATA power cable, SATA data cable, custom-cut 3M VHB double-sided mounting tape, two heavy-duty suction cups, spudger, and a 26 piece bit driver kit.
The kit does not include the extra hard drive, which must be bought separately. So if you own an iMac but require either more space or want an SSD in addition to your current drive, it seems like iFixit have the solution.
