Upon browsing the Xbox.com site, I noticed a splash page for Gears Of War 2.
Next to the trailer is a box-art cover with the RP rating.
I'm providing the full site, as well as the direct link to the image. This is a rumor, however it is on the official Microsoft site, which means it could be real.
Direct link to image: http://www.xbox.com/NR/rdonlyres/8B804457-94B4-4535-B8BB-AD3B37D5971F/0/boxepo.jpg