I've installed Snow Leopard on several 2011 Mac Minis, this is the method I use:
Connect the Mac Mini to an older Macbook with a Firewire cable (Because the Macbook only has Firewire 400 I had to use a special cable with Firewire 400 on one end and 800 on the other)
Power on the Mac Mini and hold down T to enter Target Disk Mode.
Power on the Macbook and boot from a Snow Leopard retail DVD.
Use Disk Utility to format the Mac Mini HD (be careful not to format the Macbook!)
Follow the normal Snow Leopard installation process, make sure to pick the Mac Mini hdd as the destination. Let the installation complete and the Macbook will restart from the Mac Mini hdd.
Now you have Snow Leopard installed on the Mac Mini, but it still wont boot. You need to install the 10.6.8 combo update. After installing this, restart once, then turn both Macs off, disconnect the Firewire cable and turn the Mini on again. It seems to take a while to boot the first time but after that it works OK.