I have a 2011 MacBook Pro that came with Snow Leopard. Couple of months back I upgraded to Lion. I now wish to put Snow Leopard back on a 30GB partition but no matter what I do it simply won't install. If I try and reboot holding down C it just goes back into Lion. I had to set the boot disk as the DVD and then it worked... right up until it tried to run off the disc and had a black-screen-of-death kernel panel (which incidentally had the words 'unsupported CPU' in the error message). I've tried both the disc that came with the machine and a retail version, same issue. I don't see why there would be a problem; it came with Snow Leopard to begin with!
So, is SL installed on the partition already or is that what you are trying to do? When the MBP restarts, do you hold down option to bring up the boot from selection menu?
Managed to install it by booting into target mode and installing from another machine. Any other way just wouldn't work.