It is appearing like Mayhem Fest 2013 will consist of Five Finger Death Punch, Machine Head, Job For A Cowboy, Born Of Osiris, Children Of Bodom, Emmure, Butcher Babies, Mastodon, Amon Amarth, Rob Zombie, and Behemoth.
The first date to leak is Jul 14, 2013 Comcast Center - Mansfield, MA
The Gauntlet has a pretty good source on this so it is a safe bet to say this is the Mayhem Fest lineup. More bands might be added for side stages.