I just ran across a device called a hovermower. It looks like it is basically a hovercraft push lawn mower. Instead of using wheels, it forces air down into the deck and this creates a cushion for it to ride on. It is an interesting idea. The market it as being ideal for mowing steep inclines and around ponds.
I suppose there is some advantage in not having wheels that can get stuck when you try to pull it back up the hill, but it still seems a bit odd. The video makes it look like more of a replacement for a weedeater around ponds and on slopes.