Google for some reason showed me an ad for this:

http://www.theairwheel.com/

It's the same idea as a Segway, but with no handlebar and only one wheel that fits between your legs.

To me, it looks less comfortable to stand on this thing than to walk.  It might be interesting for someone who has a physical condition that allows extended periods of standing, but limits walking.  But I have to think that's a small market.

They seem to be marketing it primarily as a fashion statement.  That might be the best strategy, given its limited utility.  After all, it's just another solution in search of a problem.  Clearly it's not the case that most people get too much exercise and need some way to cut back on walking!