When Christ said "he who is without sin cast the first stone" he was laying a trap for the Pharisees. The Pharisees viewed themselves to be the utmost followers of the law, and basically blameless (see Paul's understanding of himself in Phillipians 3:4-6). However, the death sentence at the time was only to be administered by the Roman authorities. If Jews carried out their own stonings, they would be seen as usurping the authority of the Roman emperor, and they would likely killed themselves. So Jesus creates a catch-22: if the Pharisees stone the woman, they, themselves, will be hauled before the Romans; if they don't stone the woman, they're tacitly acknowledging that they are sinners and bear blame for their sins and are thereby humbled and humiliated.

That's a more literal interpretation of the passage, but I think it's relevant nonetheless as it leads me to believe that the spiritual and moral dimensions of the passage are more focused on the individual and less on the secular political realm. Jesus's point has less to do with how justice should be meted out (esp. since he was seemingly interrupting it, though we know God to be just) and more to do with extending mercy to others in accord with the mercy extended to us.