The idea of limiting a President to a single 6 year term isn't new. In fact, the Confederate Constitution (Article II, Section 1) reads in part:

Section I. (I) The executive power shall be vested in a President of the Confederate States of America. He and the Vice President shall hold their offices for the term of six years; but the President shall not be reeligible.

If memory serves, the same applies to the President of Mexico.

Every now and then, I've heard the idea discussed here. One thing working against it is that every incoming president would automatically be a lame duck. Would that necessarily be a bad thing? Perhaps this would be a better idea if it included congressional term limits.