Recently reviewed a product called "MyMind" that has this auto-tagging feature. Evernote already has a fantastic web-clipping/saving toolset, but adding automatic tags (opt in or out by user) would be a fantastic enhancement.
100% I've just been reviewing "MyMind" product and the clipping/saving with auto-tagging is a super-impressive, helpful feature built into that product. This would be game-changing for my Tagging process in Evernote.