I have now solved the problem of being able to boot from either Snow Leopard or Lion on a new Mid 2011 iMac. This is how I did it.
1. Attach an external USB drive to an earlier Mac or borrow one from a friend. In my case I used my 2009 Unibody Macbook (which has Lion installed on it but does allow you to install Snow Leopard on an attached device) Put the Snow Leopard install disc in this machine and tell it to install Snow Leopard on your attached USB Drive.
2. Start up the Macbook (or whatever machine you are using) and get it to restart on the attached USB drive so that it boots up in Snow Leopard. Then when Snow Leopard is up and running on the attached external drive, do a software update to bring it up to 10.68 (the current version). This step is important as I found that if I plugged in the external USB drive into the iMac with only operating system 10.6 installed it would cause a kernal panic on start up when I choose to start up on the external USB drive.
3. With Snow Leopard updated to system 10.68 you can now plug it into your iMac and restart the iMac booting into Snow Leopard from the external USB drive. You do this by either going to System Preferences>Startup disk and choosing the external drive, or by holding down the option key when you restart your Mac and choosing the external USB drive.
4. The next step is to make your iMac dual boot now that you know it can run Snow Leopard successfully from an external USB drive. To do this, go into Utilities>Disk Utility and select your main drive. Select the main disk (with Lion on it) and then choose Partition and choose 2 Partitions. Drag the first partition bundary line down to make the second partition size as small as possible. In my case I had a stock 1TB drive and the smallest partition size it would allow me was 70gb. Create the second partition (assuming you have spare space on your drive to allow this to happen). This will then create a second hard drive icon on your desktop. I renamed mine "Snow Leopard startup".
5. Download the free (and excellent) utility Carbon Copy Cloner and use it to copy your Snow Leopard operating sytem from your attached USB external drive to your new partition.
6. That's it! Now you can choose to start up your iMac from either Lion or Snow Leopard using either method mentioned in 3 above.
7. As a final step I suggest your install your Rosetta only programmes in your new Snow Leopard partition so that you can access them easily and keep them all together.