Thanks for the reply. Once I partition the drive and use something like Carbon Copy Cloner, will I just be able to select either partion in the finder window and switch between systems? I apoligize for my ignorance, but I have a couple of programs that I know won't work with ML and I don't want to screw this up.
No. For that you need something like Parallels, though I am not certain it is designed for anything other than a Windows installation.
Using the solution you proposed requires you boot into the partition containing the OS X version you want. Use Startup Manager (hold an Option key while starting your Mac).
If you are really concerned obtain an external hard disk drive and install ML on it. You won't have to touch your iMac's hard disk.
A cheap way to do that is to find a PC that was discarded due to viruses or other frustrations, harvest its hard disk and return the rest to the trash. Buy a USB enclosure for the drive (about $20) and reformat it for OS X.
Parallels will work with Mac OS X Server but not the client version of 10.6.8 or earlier.
Snow Leopard client 10.6.8 (with Rosetta) running in Parallels 7 (or 8) in Lion:
[click on image to enlarge]
Full Snow Leopard installation instructions here: