Well, well, well… it seems that Ludlow Music just might have jumped the gun on their claim against JibJab for copyright infringement. It seems (as reported in this Wired News article) that Guthrie actually published the song in 1945 in a songbook, meaning the copyright expired in 1973. When Ludlow filed their copyright in 1956, the clock was already ticking on the copyright, so when Ludlow renewed it in 1984, they were 11 years too late. Good work by the EFF!!
I think now we should flood the net with amateur versions of the song, royalty free!