This was done for a contest for one of my Deviant art clubs -- [link] Basically, it was to show what Snape would see if he looked into the mirror of Erised. I did a younger Snape, when he was turning his back on his death eater ways and Dumbledore needed a way to see if he was being honest. I think it might be an interesting idea if Dumbledore looked into the mirror with Snape to see what he really wanted (notice his silhouette in the doorway) because Snape can't lie or trick the mirror. Snape doesn't desire power or riches, but simply to be redeemed. Not just that there wouldn't be the mark on his arm anymore, but that he would be pure again, all the evil he had done, all the people he has (most likely) killed, it would all be undone and washed away. Snape's deepest desire is that his conscience, and his arm, would be unpolluted. And that, folks, is why Dumbledore trust's him.

EDIT: I just ran across this passage while listening the the Goblet of Fire audio book, and I thought it sorta pertained..

“…course Dumbledore trusts you.” Growled Moody. “He’s a trusting man, isn’t he? Believes in second chances, but me? I’d say there are spots that don’t come off Snape, spots that never come off, do you know what I mean?” Snape suddenly did something very strange. He seized his left forearm convulsively with his right hand, as though something on it had hurt him.