I was going to the bank at around 11:00a, and a guy passed me right at Christy & Holly Hills in S. STL City on what looked like a Puch front end welded to the back of a garden cart type of thing. It had big spoked wheels in the back and a big basket. He was only going about 20. Weird but very cool, didn't get a chance to stop & talk. Looked pretty stable going around the turn, though, the back end was pretty wide.
In MO, any two- or three-wheeled vehicle <50cc, 2bhp and doesn't exceed 30mph on a straightaway is considered a "motorized bicycle" and therefore does not even get a title issued for it. I'd never seen one take up the three-wheeled part, though...