During the recent MI6 Game Marketing Conference, ESRB President Patricia Vance addressed the topic of rating downloadable content. It's only been i recent years that downloadable content has evolved, originally downloadable content was basically just modifications made by an enthusiast. Now however developers alike at extending gameplay beyond the box you purchase. Oblivion and Grand Theft Auto IV are huge games, and both have downloadable content coming soon, they have helped bring this 'downloadable content rating' problem back into the focus for the ESRB.

Simply, developers must "keep the content in the downloadable product consistent with the core rating." What does this mean? It means that the content put up for download must not contain content higher of that on the original rating. For example; you could not have a downloadable package with sexual content for an M rated game that only had violence and horror content according to the rating on the box. If the content is changed, then the ESRB would force a re-rating, which would cost the company money no doubt (for updated boxes I would presume).

Source: MTV via Wired