Well, it's not a perfect material by any means, but iPhone 3G is the first mobile Apple gadget I've owned that won't scratch as long as you're moderately careful. If you've ever had any of the iPods with a chrome back (1st gen Nano, 1st gen Touch, 5th gen video iPod etc) you'll know that they will scratch from ANYTHING. They'll scratch if you place them carefully on a tabletop and accidentaly nudge them half an inch. They'll scratch if you wipe them with a cloth. Basically, if you have lint in one pocket and a chrome-back iPod in the other, the lint will somehow manage to scratch the iPod by association.
I've had a white iPhone 3G for five or six weeks now and there's not a single visible scratch on it, even though it's 'naked'. It's possible that the black one scratches easier, I wouldn't know, but it's still pretty damn good for an Apple product. I would worry more about the chrome frame around the display, I have a feeling it's the same old crap they used on the back of old iPods, and if so, it will look like sh*t long before you notice a single scratch on the plastic...