Just want to point out that being rated by the ESRB is not free. That's why Binding of Issac is not rated, it means just that...it has not been rated yet. It has nothing to do with the game's content. Had Nintendo agreed to allow the game to be published, McMillien would have had to submit the game for a rating and I'm sure WOULD have done so.
Since Nintendo rejected the game out of hand, there's really no reason for him to bother unless he talks to Sony or Microsoft and has better luck.
As far as what rating it would receive, I'd like to point out that Dante's Inferno received an M rating despite having bare breasts, religious content and far higher realism than Binding of Issac. It seems very likely to me that Binding of Issac would receive either a T or M rating.
To those saying the game is merely about 'shock' value, I disagree. The game is legitimately fun. I'm not easily shocked, nor was I shocked by this game, but I still enjoyed it a lot. It's a good game. The greatest weak point of the experience is probably the controls. The gameplay itself is a mix between a rouge-like, zelda dungeon, and shooter. It's a pretty fun combination of elements and for the price (cheap) provides a fair bit of gameplay due to the rouge-like nature.
Personally I don't think Nintendo should make themselves gatekeepers of what issues and concepts a game can touch on. I understand that they need to ensure a game's quality (that it works and is playable). I will even accept them keeping an AO game off their store.
Where I take exception is when they start trying to police thoughts and ideas. That's what's really happening here. Much like I don't want a bookstore to block sale of thought-provoking, but controversial, books like Catcher in the Rye nor do I think Nintendo should be blocking the sale of a legitimate (like it or not) and thought provoking (agree with it or not) game.
Thought, debate, and free discourse is being crushed down and that isn't cool. Nintendo has the right to do what they're doing, but they don't have to and they shouldn't.