a_caspis

Because they are easier to manufacture than other shapes which will not fall through the hole either: http://en.wikipedia.org/wiki/Reuleaux_triangle

Want to attract nerd tourists ? Build your city with Reuleaux manhole covers in the streets and Penrose tilings on the walkways.

Did it fall out of grace because it was vulnerable to cross-site attacks ? ( http://en.wikipedia.org/wiki/Cross-site_request_forgery )