Here's how I managed to reconfiguring a new iMac that has Lion preinstalled for Snow Leopard. Took 2 calls to TechSuppt who told me it couldn't easily be done. I now have 2 partitions, one with SL, one with Lion.
What I used:
New Mac: mine is an iMac 3.4 i7 - July 2011
Old mac with latest Snow Leopard installed: mine is an iMac 2.8 24"
External hard drive (firewire or USB - I used the latter) that has enough space for all the stuff from the old Mac.
Snow Leopard install disk (full version, not the one that is dedicated to a particular computer)
Part 1: Creating a Snow Leopard disk usable by the new computer.
1. Use an external hard drive and a Snow Leopard Install disk to create a bootable external drive containing Snow Leopard.
2. Boot up the old Mac using the external drive (by holding down the Option key) and do a System Update to ensure that the latest version of Snow Leopard is on the external drive (anything earlier than 10.6 won't work on the new Macs).
3. Use Migration Assistant to transfer all of the data from the old Mac's internal drive to the external drive
4. Power down the old Mac and move the hard drive to the new Mac
5. Boot up at the new Mac while holding down the Option key. When the boot-up disk options display on screen, select the external drive as the boot-up disk.
The new Mac should now boot up with Snow Leopard and all of the files and configuration from the old Mac (subject to the limitations of Migration Assistant). If using an external drive is OK, then stop here.
Alternatively, to do away with the External drive altogether, here's how install all this on the internal drive:
1. Boot up the new Mac using its internal drive (i.e. Lion): ideally this won't have any data on it already
2. Launch Disk Utility
3. Select the internal hard drive (root directory) and select Partition Drive (can only do this from within Lion, not from Snow Leopard).
4. Add a new partition and set its size according to how much space is needed both for Snow Leopard and home files plus any extra for the future. Call the new partition by a recognizable name, e.g Macintosh HD SL
5 Select Create Partition and check that the partition is of the correct size.
6. Reboot the new mac from the External Drive (Option key)
7. Use / download a cloning application (I used SuperDuper) to clone the contents of the external drive to the new partition, making sure you make the partition bootable.
At the end of the process the new Mac will have 2 partitions on, one with Lion, the other with Snow Leopard. It may be possible to avoid the partitioning and copy the External Drive (using SuperDuper) to the single original partition, thereby wiping out Lion, but I didn't try that (I felt it was too risky and I only need Snow Leopard for a few applications). Of course more preferable, but totally outside my control would be for Lion to support Rosetta, or some other emulator… In the meantime, this works, at least on my machines, though with the hassle of duplicate set ups. Good job it has a 1 TB drive! The bloody thing is half full already...