The idiom refers to the false denial of desire for something sought but not attainable. However, in recent times it is applied to all denials of desire. For example, my decision to remain miserable in my job instead of accept a seemingly better offer attracted an allegation of 'sour grapes' from my family. If I decide that I don't want a flat screen LCD, people form the automatic assumption that I: a) can't afford it, b) deny my desire as a result; sour grapes.

 

Modern applications of 'sour grapes' fail to examine whether the denial of desire is indeed false. I believe that society has been trained to view all desire as basically good. Aspiration and the fulfillment of our ambitions - no matter how pedestrian - is what keeps our economies going. If we think positively, we will attract what we want.

 

Above lies the hidden paradox with new-thought positive attraction theory. If we can not, for whatever reason, attain our desires then it follows that we never really wanted it in the first place, and that therefore denial of the desire is true. Modern 'sour grapes' also fails to account for the circumstance in which the fox, having gotten far closer to the grapes than we have, saw that they were indeed sour. Just because we can, doesn't mean we should.

 

Wikipedia has a very good brief on the idiom and its source fable, 'The Fox and the Grapes'.