[Dave Gage] “1. I have an early 2011 17″ i7 that did ship with Snow Leopard.
2. My wife has my older 2009 MacBook Pro Core 2 Duo 2.53 15″. I believe this originally shipped with Leopard.”
That’s why you can still boot Snow Leopard. I think the idea in question was installing Snow Leopard on a Mac that originally came installed with either Lion or Mountain Lion.
I have the same Mac you do (17″ Early 2011) and I’ve easily gone back and forth between Snow Leopard, Lion, and Mountain Lion (though settled on ML…working very well for me).