I know that I have seen these before (San Francisco I think), but was recently reminded of their genius when I visited an Aldi grocery store in Dallas a few weeks ago. The parking lot had not a single, stray cart meandering about, doing away with dinged car doors and the staff required to corral those suckers in the oppressive heat.
aneelee 
I always return my cart, but unfortunately was the victim of one hitting my car while parked; Great idea!