The first batch of ALU iMacs shipped with Tiger, so it's definitely a possibility.
The question is, if the ALU iMacs that shipped pre-loaded with Leopard, can go back to Tiger. Apple usually makes it so machines can't go back to an OS that came before it. Since there was an overlap between OS's for this line of hardware, unless Apple altered the bios in machines that shipped pre-loaded with Leopard, you should be able to go back to Tiger.
So, anyone try/go back to Tiger on a computer that came pre-loaded with Leopard?