You can do it fairly easily via target disk mode if you have another Mac running 10.6.8.
- OS X 10.6 retail disc
- OS X combo update 10.6.8
- firewire cable (400/800) depending on the mac you're using to target the machine you want to downgrade
Connect the computers via firewire cable, hold down "T" on the computer you wish to downgrade while starting up. There should be a firewire icon on the screen of the downgrading computer, so you know it's connected properly.
From there, format the hard drive in the computer you wish to downgrade - you should be able to select it using Disk Utility. Make sure there's 1 partition and format it. Then, insert your Snow Leopard retail disk and install it on the freshly formatted hard drive.
Once Snow Leopard is done installing, it will restart and boot up using the freshly downgraded computer's hard drive. From there, you can install the Mac OS X 10.6.8. combo update - either have it already downloaded and on a USB drive, or you can download it from apple's site. Once that installs, you should be able to disconnect the computers and boot normally into Snow Leopard on the early 2011 MBP.
It worked perfectly for me. Good luck.