After the Hot Coffee debacle that occured recently, the ESRB has told game publishers in a recent letter that all hidden content in games must be disclosed during the ratings process. The ESRB has always had rules that all cheat codes and Easter eggs on game discs must be mentioned, but the letter is meant to crack down and reinforce the rule.
"Coding around scenes, images, or similar elements that might be pertinent to a rating assignment does not render this content irrelevant from a ratings standpoint. If a publisher wishes to "edit out" pertinent content from a final product, it must remove the content from the disc altogether. If that is not feasible, the pertinent content must be disclosed to the ESRB during the rating process so it can be taken into account in the assignment of a rating," the ESRB wrote. "If you fail to notify us of previously undisclosed, non-playable, pertinent content by January 9 (2006), and such content becomes playable through a subsequent authorized or unauthorized release of code to unlock it, rendering the original rating assignment inaccurate, punitive in addition to corrective actions may result."