Some folks see grafitti and think "Now that's art" and others see it as the defacement of property. Personally I find even grafitti I like to be a defacement of someone's property unless of course the property owner invited the artist to work his magic. That's why in the realm of "unapproved" art I prefer something that's creative, easily removable and inherently temporary. Here's a great example:

Then there's the stuff that's approved and is anything but art. Like cell towers disguised as trees.