On Playstation Nation's weekly podcast, industry expert Andrew Yoon from the Joystiq network sheds some light on the industry practice of early exclusive game reviews. He mentions that Rockstar blacklists review sites, such a Joystiq, that give them unfavorable reviews/previews and have a direct hand in the content of the written reviews. This may explain IGN's perfect score for the game since they had the exclusive first online review of the game. Joystiq is still blacklisted for an unfavorable preview of Liberty City Stories for PSP.
Listen at at around 10:00 minute mark for the question that garners this reponse from Yoon.