Art or Porn?

What distinguishes one book, film or picture as art from another that's merely pornography?

Some friends and i are going to watch the Oscar-nominated movie, Nine, this weekend (my review of Nine will be up on my Heartflicks Film Review Blog on Monday), despite some people's reservations that it's "just soft porn" and "a bit dodgy". These misgivings were countered with comments that it's actually art that inspires and demands serious talent. While you can't really judge much from the previews, in some ways it does seem like a musical excuse to be smutty. But you can't really tell until you've seen it – and if it's been made by some seriously gifted people and has been nominated for awards, it surely can't be just bad?

Which makes me wonder: what determines the difference between art and pornography? What makes one film a cultured flick and another a blue movie? What makes one nude portrait an artwork and another a dirty picture?

Is it the motivation of the producer for creating it? Is it the quality/skill/technique of the execution? Or is it the intentions of the audience in watching it?

So what do you think makes it art or porn?