The above [now below as this is the 'accepted' answer] instructions will install Snow Leopard - but not technically a clean install.
To do so follow the above steps but enter Disk Utility after choosing the language in the installer. You can wipe it from there.
Stolen from Macs.com
the Snow Leopard installer will ask which language you want to use as the main language. Make your selection and click the right arrow key.
The Install Mac OS X screen will display. Click the ‘Utilities’ button.
In the Apple menu bar, select ‘Disk Utilities’ from the Utilities menu.
Disk Utilities will launch. Select one of the following instructions, depending on what you wish to do.
Erase a Volume. Use these instructions to erase an entire volume. Be sure to back up all of your data first.
Format a Hard Drive. Use these instructions if you wish to erase an entire hard drive, including any volumes/partitions it may contain, and not create any new volumes/partitions. Be sure to back up all of your data first.
Partition a Hard Drive. Use these instructions if you wish to erase and partition a hard drive. Be sure to back up all of your data first.
When you have finished using Disk Utility, select ‘Quit’ from the Disk Utility menu.
You will be returned to the Snow Leopard Installer to continue the installation.
Legally, you can only run OS X Server in a virtual machine for 10.5 and 10.6. With Lion, they allowed regular OS X installations to run in up to two virtual machines. VMWare Fusion does support Leopard and Snow Leopard Server virtual machines (but not yet Lion), but I don't know if it will let you use the same installation both booted from disk and in a virtual machine.
Best Answer
General rule is no Mac can run an OS older than itself. There are no drivers for hardware that wasn't yet invented. The 2011 Macbook Pro runs OS X 10.7.2 at minimum.
You could normally run an older OS in a VM, but Leopard isn't licensed for virtualisation (see What are the technical and license restrictions around virtualization for Leopard, Snow Leopard and Lion?).