Naked iPhone users would be awfully cold this time of year. I avoid belt buckles when playing acoustic guitar, but other than that, I'm rarely naked.
Seriously...... I've been using mine with only a screen protector and a sleeve without problems. I have developed a habit of not wearing belts because I play classical guitar, but I could see belt scratches occurring with a naked iPad. Belts, rings, metal watches, car keys in front pockets, metal buttons, are all things that will scratch or crack wood on fine guitars, and I could see some of those causing scratches on an iPad. My wife drags a metal band Seiko watch across the area around her rMBP's keyboard, and that's like nails on a chalkboard.
EDIT: You'll want to be careful when friends use you naked iPad. I have guitars that range from $400 to $4,000 dollars, and let my sister's dufus boyfriend play one of my cheaper guitars. I didn't notice the ring he had on his thumb, until I saw the dent it put on the guitar's neck. He's a bass player, so I should have known better. This is especially true with the new iPad, as people will be flipping it around to look at it from every angle.