It's not the dog's fault, he's just a dog. YOU go shit on the neighbor's porch. That'll show 'em.
Yeah, if your kitteh walks on your neighbors car and your neighbor doesn't hurt your kitty, count your blessings. Many people would think nothing of hurting your cat for such a "horrible" thing (some of us probably know people like that.)
I'd clean it up and move on, like jinx said. If it happens again, I'd explain the problem to the neighbor.