I've only ever seen two. I didn't even realize you could do this to a bike until I saw the first guy buzz past on the other side of the road. Then I thought that looked a lot better than walking so I started researching. (I don't really remember cause at the time I knew nothing, but I think it was a happy time). Second time was when I was sitting on the grass in an out of the way area of my university's campus. A guy walked by, a professor I think, with a tiny, puttering engine in idle on his bike.
Now that I've got my own, gets a few stares and questions, so I don't think most people have seen them much, if at all.