Infinte Resistors

This morning in Intro to Circuits, two students had the following problem waiting on the board for our professor, Shealy. He just looked at it and said, "Thats uh,.....(short pause)... not on the homework." Although he had a funny response, I was really hoping to get some insight. I first saw it on the Google Labs Aptitude Test, and then later on XKCD, but haven't seen it solved yet. I await that day....