ESRB's Official statement:
 
Earlier this week we learned about a hack into the code of the PSP and PS2 versions of the game that removes special effects filters that were put in place to obscure certain violent depictions.  We have investigated the matter and concluded that unauthorized versions of the game have been released on the Internet along with instructions on how to modify the code to remove the special effects.  Once numerous changes to the game's code have been made and other unauthorized software programs have been downloaded to the hardware device which circumvent security controls that prevent unauthorized games from being played on that hardware, a player can view unobscured versions of certain violent acts in the game.  Contrary to some reports, however, we do not believe these modifications fully restore the product to the version that originally received an AO rating, nor is this a matter of unlocking content.

Our investigation indicates that the game's publisher disclosed to the ESRB all pertinent content in the authorized Mature-rated version of Manhunt 2 now available in stores, and complied with our guidelines on full disclosure of content. 

What parents, and indeed all consumers, need to be aware of is that computer software and hardware devices are susceptible to unauthorized modification.  Parents should be cognizant of whether or not their children are engaging in unauthorized modification of their games, consoles or handhelds, as those modifications can change game content in ways that may be inconsistent with the assigned ESRB rating. That being said, the vast majority of consumers have not made the unauthorized modifications to their hardware necessary to view the content at issue. 

ESRB president Patricia Vance said, "Manhunt 2's rating makes it unmistakable that the game is intended for an older audience.  The unauthorized hacking into the code of this game doesn't change that basic fact. Parents need to be vigilant about monitoring what their children are downloading on the Internet and ensure that they are not making unauthorized and oftentimes illegal modifications to software and hardware that remove the controls the industry has so diligently put in place for their own protection."