http://en.wikipedia.org/wiki/Godwin's_law

"Godwin's law applies especially to inappropriate, inordinate, or hyperbolic comparisons of other situations (or one's opponent) with Nazis – often referred to as "playing the Hitler card". The law and its corollaries would not apply to discussions covering known mainstays of Nazi Germany such as genocide, eugenics, or racial superiority, nor, more debatably, to a discussion of other totalitarian regimes or ideologies,..."