I am planning on doing this as well, although with a 2009 you have to sacrifice the Superdrive if you want to keep the original HDD.
I've been linked to this part:
http://www.ifixit.com/Apple-Parts/12-7-mm-SATA-Optical-Bay-SATA-Hard-Drive-Enclosure/IF107-081
Although the page does not state an iMac, many old tutorials are going to this page.
I currently have a third party SSD within my iMac. I would like to install the original HDD inside, and keep the SSD and the superdrive. Is this possible?
I have the Late 2009 27" model, i7 @ 2.8 GHz.
I have that same model and am intersted in an SSD in place of the HD. Did you do the install yourself? What make/model did you install? Any issue regarding fan operations?