http://www.gamasutra.com/php-bin/news_index.php?story=6490
Basically the ESRB is doing a massive audit of games for hidden extras that caused the hotcoffee scandal, and then this fun little line comes along.
Finally, the ESRB addresses third-party 'mod' content which could potentially change the game's suitability, but was not inserted by the game's developer, commenting: "ESRB remains concerned about third party modifications that undermine the accuracy of the original rating, and we are exploring ways to maintain the credibility of the rating system with consumers in light of modifications of this nature."
I'm off to write an email to the ESRB letting my feelings being known to them.