There should be no reason why you couldn't install Mac OS X 10.6 (Snow Leopard) on your new iMac.
What you'll need to do to make it work is format the disk using the "GUID Partition Scheme". To do this, boot your iMac using the Mac OS X 10.6 Install DVD by placing the disc in your computers optical drive, then restart your Mac and immediately hold down the "C" key.
Once your Mac has booted from the disc you'll be prompted to choose a language. Proceed to do so, then click on Utilities, and from the drop down menu that appears choose "Disk Utility".
In the Disk Utility window, select the disk you wish to re-partition (which will be displayed as the size and drive type, not the volume name which is typically "Macintosh HD"). Click the Partition tab, and select the number of partitions you wish to create. Now click on the Options button, and choose "GUID Partition Scheme", then click on OK. Now click on the Partition button to begin the partitioning process, which only takes a few minutes.
(For more information on partitioning your hard disk, please see
http://support.apple.com/kb/ts1600).
You can now proceed to quit Disk Utility and follow through with installing Mac OS X 10.6.
For more information on how to install Mac OS X 10.6, please see
http://manuals.info.apple.com/en_US/Snow_Leopard_Installation_Instructions.pdf and
http://support.apple.com/kb/HT3910?viewlocale=en_US.
Hope that helps and have a great day!