How should Christians portray sin in their art?

How should sin be portrayed in Christian stories and novels?

Author L.B. Graham has posted an essay arguing that the depiction of sin in Christian fiction must be more than moralism. The full essay is worth reading, but here's a short excerpt:

I’m constantly surprised at how often fictional stories are judged to be Christian or not, based more or less on how well the characters behave themselves. Of course it is true that morality matters - God has taken great care to expound in some detail the moral laws which flow from and are an extension of His own character. It is also true, though, that the Bible itself is full of flawed men and women whom God used almost despite of rather than because of their moral triumphs. However, when Christian writers incorporate flawed heroes into their stories, men and women with moral failings of any significance, they are often left open to charges of having given dubious testimony to their Lord.

He goes on to suggest that Christians ought to resist the initial impulse to praise or condemn a story's moral quality based on the morality of its protagonists.

It's a good essay (and has applications to Christian interpretation of other artistic media as well, particularly film). I think most Christians would agree that it's possible (and even desirable, for the sake of believability) to portray flawed characters or sinful acts in fiction without actually condoning those behaviors. But looking at the variety of Christian film criticism available on the web, it's clear that we all seem to "draw the line" at different points when it comes to the acceptable portrayal of sin in our stories.

How do you approach this issue? A few questions to get you thinking:

• Does a Christian writer have a moral obligation to condemn sin portrayed in his or her story? ("Condemn" here might mean explicitly rebuking it in the story, or depicting the sin's consequences in a way that casts it in a negative light.) Would you be bothered if sin in a Christian's story (say, sexual sin committed by the protagonist) is not addressed?
• Does a Christian writer have an obligation to describe—or not describe—sin differently than a non-Christian might? Should a Christian writer shy away from graphically describing certain sins—sexual sin, violence, etc.? What about less "tangible" sins, like greed or envy?
• What's the best portrayal of sin you've encountered in a novel (Christian or otherwise)? What makes that portrayal so effective?

(Hat tip: Looking Closer.)