That's true, in the original fable the grapes were inaccessible to the sole character (the fox) and the moral was just that "IT IS EASY TO DESPISE WHAT YOU CANNOT GET." Colloquially, however, "sour grapes" is often repurposed to describe the rationalizations of a sore loser, that is, when someone else CAN get the grapes but you cannot. That is clearly ellyagg's usage, and as in the cognitive dissonance of the original fable, there is an implication that the loser would change his tune if he were to gain access to the "grapes". ANH disagrees, asserting that the grapes are genuinely sour in this case.

From http://www.bartleby.com/17/1/31.html (and it's well into the public domain):

ONE hot summer’s day a Fox was strolling through an orchard till he came to a bunch of Grapes just ripening on a vine which had been trained over a lofty branch. "Just the things to quench my thirst," quoth he. Drawing back a few paces, he took a run and a jump, and just missed the bunch. Turning round again with a One, Two, Three, he jumped up, but with no greater success. Again and again he tried after the tempting morsel, but at last had to give it up, and walked away with his nose in the air, saying: "I am sure they are sour." "IT IS EASY TO DESPISE WHAT YOU CANNOT GET."

Upvoted you both. I did do a little research before interpreting the original "sour grapes" that way. I, too, think there's room for debate about that interpretation, but I stand by it. Thanks for the exposition.