Trent tried to mark indoors once - at Petsmart, over where another dog also just marked. That was 7-8 months ago, when he first started the behavior. It was corrected, and every time after that I watched him like a hawk when he was out with me to prevent inappropriate marking. He hasn't done it again, and he goes out with me, whether to Petsmart/Petco, a supply store, appliance store, video store, etc., a couple times a week.
But I completely agree that some dogs are, indeed, more adamant about it than others, when it comes to marking behaviors and humping. And many times, neutering does cure it. I just don't have a problem personally with training as an alternative. I will admit, though, it can be extremely tough to train some dogs to stop.