Try searching for articles about Wal-Mart. The CDs they sell are often slightly different than other retailers. Case in point, I notices one of Eminem's CDs at Wal-mart had the track listing altered to not show the swear words ('*' instead of letters). The same CD at a locally owned store had a non-edited track list. Wal-Mart has also asked computer game makers to tone down the violence in some games (sorry, don't remember any examples). The reason I mention Wal-Mart at all is it alone accounts for such a huge percentage of retail sales in the US that if manufacturers don't do what Wal-Mart wants, it can destroy a product by not carrying it.
The video for Mystical's song "Shake Ya Ass" was submitted as is to BET, but MTV got "Shake It Fast". Mystical (or his producers) self sensored the song because he didn't expect it to get as much airplay if they didn't (not that that video left any doubt as to what was being shaken).
Look up the PMRC (Parents' Music Resouce Center). You can start with
http://archive.aclu.org/library/pbr3.html.