Some people here suggest that the ESRB is complicit in killing off this game - I think that's a weak argument at best. There are political concerns for the ESRB, sure, but ultimately they would be "doing their job" marking the game AO, whether or not it hit the market.
Nintendo and Sony have killed this game by refusing to sell "AO" games on their system. There's little to contrast this with in media, as other outlets (tv, film, music) allow for easy entree by independents. People can make and distribute their own shows, albums, and movies, with no official involvement at all. For games, however, the technological limitations (and, more importantly, costs) get in the way. You can make PSP homebrew, I suppose...but nothing of Manhunt's magnitude or design.
There's no recourse, and, of course, going "DIY" doesn't change the issue of censorship that the console makers all share.
I really doubt this is a publicity stunt, since the game, to be salable, will ultimately have to be rated "M." By doing that, you immediately cast doubt on the "authenticity" of the game (this must be revised or have content deleted to have gotten this rating) if it is released. That's bad publicity.
This must be this decade's "Thrill Kill." I got to play that game eventually, and boy did it suck. That, of course, is not the point.