Okay, so I'm not going to say they were trying to make a comparison to Christ with The Dark Knight but Christ is the standard of all heroes and I'm going to see where The Dark Knight stacks up - and it stacks up very well.  Much better than messianic characters like Neo from the Matrix or William Wallace from Braveheart.  And... I'll keep this short.

This amazing analysis contains spoilers, so don't read it if you care about learning about what happens in the movie.

Notice the Joker is not a typical bad guy.  He's not out to destroy good, he like Satan, is out to turn good into evil. 

The movie said that Harvey Dent was the symbol of the good in Gotham.  So what does Joker do?  In the hospital he doesn't kill good, he tempts good and turns it into evil.

Now bring in Batman.  In the dialogue it is constantly asked, "how does Batman become more than a hero?"  Well somehow either intentionally or unintentionally the writers came very close to the Christ example.  The movie ends with Batman saying something to the effect - that to become more than a hero, he must become whatever Gotham needs him to be.  In this case Batman had to assume the sin of Harvey Dent, as to save the city.  In the same way Christ had to assume our sin to save us.

I could go much deeper into this, like how Joker tried to turn the good people on the ferryboat into murders, but that would just make this article way longer that it needs to be.  You get the idea.  Obviously the comparison only goes so far, but it went way further than any other movie I can think of at the time because of the 'becoming sin for others' concept.  Batman just didn't die during that act.  If he did then he'd be much closer to the standard that Christ set.