Camassia points out a difference between how Jesus and St. Paul taught charity toward enemies. She refers to these two passages:
But I say to you, love your enemies, and pray for those who persecute you, that you may be children of your heavenly Father, for he makes his sun rise on the bad and the good, and causes rain to fall on the just and the unjust.
Beloved, do not look for revenge but leave room for the wrath; for it is written, "Vengeance is mine, I will repay, says the Lord." Rather, "if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals upon his head."
They seem to be arguing for pacifism from opposite directions. Paul is saying, don't crush your enemies because God will crush them for you. Jesus seems to be saying, God doesn't crush your enemies, therefore you shouldn't either. Paul's reasoning is more in line with the Old Testament theme that Yoder points out, though Jesus' point is not without foundation. Particularly apposite is the book of Jonah, where the hero explains why he wouldn't preach to the evil Ninevites: "I knew that you are a gracious and compassionate God, slow to anger and abounding in love, a God who relents from sending calamity." (The fact that Jesus refers to his three-day entombment as "the sign of Jonah" may be for more reason than the fish episode.)
St. Paul's "burning coals" bit was lifted from Proverbs:
If your enemy be hungry, give him food to eat, if he be thirsty, give him to drink; For live coals you will heap on his head, and the LORD will vindicate you.
The NAB has this note on "live coals": "either remorse and embarrassment for the harm done, or increased punishment for refusing reconciliation."
I'd always thought of the coals as a psychological punishment for your enemy and a psychological reward for yourself. But as the NAB note suggests, burning coals can also signify divine punishment (in, for example, Ezekiel 10).
So the proverb (and St. Paul's use of it) can be understood as meaning that if you love your enemies, you place them under divine judgment. If they accept and reciprocate your love (as the Ninevites accepted and acted upon Jonah's prophecy), then they are saved. If not, not.
On this reading (well, maybe "accommodation"), loving your enemies is a way of extending the New Covenant to them. If they love you back, then they are keeping Christ's commandment to love their enemies, and He and His Father will come and dwell with them. If they don't love you back, the LORD will still vindicate you (and perhaps we could say your love will return to you).