I just sold my 1 yr old iPhone and got a 3G. The 1 yr old phone was flawless. It was carried in my jeans or other pants/shorts pocket, sometimes alone, sometimes with keys and change. Mostly just alone in that pocket.
Secret to keeping it flawless, not a scratch, nothing - BestSkinsEver.com skin for my iPhone (rear skin only $7 shipped, full skin $10 shipped) and a PowerSupportUSA.com Crystal Film Set for iPhone, now avail for 3G ($15 for pair of screen protectors - avail at Apple Stores).
I even dropped my iPhone face up and face down several times from my lap getting out of my car on to gravelly concrete and asphalt streets, perils of BT headsets, you forget where your phone is, and not a mark, not once.
The skin is that same 3M stuff they use on clear bras for cars. Thin enough it still fit in my docks though.
I used the Crystal Film on the screen b/c even though the Full Body Skin comes with screen component, the skin is a bit grippy/tacky which is great for not having your phone slip from your hand, I wanted a smooth feel to the screen, and the Crystal Film feels just like the naked glass.