According to an official statement from Brownmark Films, the company, best known for the above completely idiotic video and Internet phenomenon from 2007, has decided to file a lawsuit against Viacom International, Comedy Partners, Paramount Pictures Corporation, MTV Networks and South Park Digital Studios LLC, with accusations of copyright infringement. The alleged infringement is from a South Park clip from the "Canada on Strike" episode, originally aired in 2008.
What's confusing about the situation is that the song was initially licensed to South Park, so use of the song is not what's being argued here. Instead, it's the video content. More confusing is the fact that in a posting on the Samwell's (the artist in question) website, he seems rather excited about the parody, saying,
...after the video had already been downloaded nearly nine million times, an episode of South Park aired that revolved around the What What phenomenon and featured a shot-for-shot recreation of the video...
So the issue at stake here isn't whether South Park was allowed to use the song -- they were -- but Brownmark Films never gave them the rights to the content of the video. It can be safely assumed that South Park's lawyers are going to come at this straight, claiming it's a parody of the original, not a copy.
This has been an issue since the first-ever parody was done, so the precedent is pretty clear. What is unclear is why Brownmark Films decided to wait two years before filing the lawsuit. If a vast array of Weird Al parodies are any indication, however, they don't stand a chance against Viacom et al.
If you've somehow missed the thousands of SNL sketches and millions of parody songs out there in the world and don't understand the difference between parody and copyright infringement, here's a primer: Parody is often protected under the "fair use" factors found in the Copyright Act, which assumes that the work doesn't negatively affect the market value of the original content, is some type of cultural criticism, is entertainment, and doesn't copy too much.
There's more, of course, but the arguments above are some of the more common ones brought up in these cases. In this one, the original song seems like a parody itself, so it's difficult to really say what the courts will decide.
Regardless, if this goes to court, it will be amusing to read the transcripts of Viacom's lawyers arguing that "What What (In the Butt)" is protected under "fair use" as "criticism, comment, scholarship, research, news reporting or teaching."
Here's the South Park version, in case you missed it.