I know parts of Song of the South have been issued in the past - most notably some of the musical segments for their "Sing-a-long Songs" series. I also know that the animated Br'er Rabbit segments* have been aired on various Walt Disney programs, and I think compiled for a home video release. But I don't think the movie as a whole has ever been released. If I'm wrong, hey, I'm sorry. Wikipedia backs me up - though of course, wikipedia is only a slight bit more factual than a rumor about a 5th WDW theme park spread by a monorail driver. But snopes also backs me up and they normally know their stuff
http://www.snopes.com/disney/films/sots.asp
I also know that it has been released in other parts of the world; I believe, when you see bootleg versions of the movie being sold on ebay, they're usually copies of a video released in England (which would explain why there's no dubbing into other languages, or no foreign language subtitles.
*For those not in-the-loop, most of Song of the South is a live-action movie taking place shortly after the Civil War. One of the characters, Uncle Remus, is a local stoyteller who regales the children in the movie, most notably the protagonist, Johnny, with stories about Br'er Rabbit, Br'er Fox and Br'er Bear. The storytime portions of the movie are animated. Usually, if you find anything Song of the South related on a Disney video, it's the animated sequences from the movie, and Uncle Remus (James Baskett) singing Zip-a-Dee-Doo-Dah, which is a combination of live-action and animation.