Cables for downtubes?
 

Anybody remember bikes in the early 90's (maybe 90-92-ish) with a cable or wire instead of a downtube. I was just thinking about those recently. I saw them in a bike shop around that time. I've never seen one since or ever on the road. Doesn't seem like it would be stiff enough.

Slingshot.

http://www.slingshotbikes.com/