How can I downgrade an iMac12,1 from Lion to Snow Leopard?
You can do this if you have an older Snow Leopard-compatible Mac.


Put the new iMac into Target Disk Mode and connect it to the older Mac with a Firewire cable. Its hard disk should mount on the desktop of the other Mac. (Hold T at startup to enable Target Disk Mode)
Boot the older Mac from the SL DVD that came with that Mac and install onto the new iMac's hard disk.
Before Rebooting the iMac Download and install the Mac OS X 10.6.8 Combo Update while booted from the older Mac's normal operating system. Make sure to target the new iMac from within the Installer.