Turns out Woody Guthrie lifted the melody of "This Land Is Your Land" essentially note-for-note from "When the World's on Fire," a song recorded by country/bluegrass legends, The Carter Family, ten years before Guthrie wrote his classic song. Here's a short snippet (380k mp3) of the song (the song can be found on the box set, The Carter Family: 1927-34). You don't need to be a musicologist to hear what we're talking about.
Now we've got nothing against Woody's borrowing. In fact, it's a part of the "folk process" that Woody himself championed. I can't imagine that The Carter Family minded.
But in the letter threatening copyright litigation over JibJab's animated political parody, "This Land," Ludlow's lawyer goes out of his way to attack JibJab for copying "the entire melody, harmony, rhythm and structure of the [sic] Mr. Guthrie's song."
Er, sorry there Ludlow, but actually, the entire melody, harmony, rhythm, and structure of "This Land Is Your Land" doesn't belong to you. And I'd like to think Mr. Guthrie would never have claimed credit for them, if he were still alive to ask.