To be honest, as me, it would depend on who I am dining with. If I was alone, I would go get it and enjoy every bite of it. If I was with my gf, she would never permit me to go near it. If I was with my family, I would get it and share it with all of them..
I would think "It's a trap!" and I wouldn't go for it. There's just something about eating food that was on a stranger's table that doesn't appeal to me. You don't know what's been done to it.
But morally speaking, I see nothing wrong with eating it if it was left there. It's already been paid for and the one who ordered it obviously didn't want it. So as far as I'm concerned it's up for grabs.
The only problem I foresee the restaurant having is that they might not let you eat it for legal reasons. If in fact it was tampered with and you ate it and somehow got sick, they'd have a potential lawsuit on their hands. If they don't let you eat it, it's because they have to cover their asses as a business.
I'll just leave it, since I don't eat desserts at restaurants. I'm usually too full after the main course (my stomach can't exactly take in a lot of food). And even if I wasn't full I'd still leave it alone since I could just get my own.