Tyler Galpin with an incredibly detailed look at how one goes about installing an SSD in their 2010 27″ iMac, while maintaining the stock HD:
>First of all, I did this and it was a hugely ridiculous process. I learned a lot of things the hard way as I went along with the install, so hopefully this guide will help some of you through the process. Details: I have a mid-2010 27” iMac and installed this SSD to work alongside my HDD which was already factory installed in the iMac. 

Great source of information if you want to make this upgrade.