before I got my iPhone, I was going crazy researching cases, but eventually I found a thread over at iLounge where a bunch of people talked about how going "nude" with the iPhone was just fine. So thats what I'm doing, and its amazing how scratch resistant it is. I've dropped it about three times at this point (from lower heights onto softer surfaces, i.e. carpet, not concrete) and have had no issues. Sure, the screen gets grimy and fingerprinty, but this doesn't inhibit usage at all for me, and a quick wipe from a soft cotton t-shirt cleans it perfectly.
In other words, I opted to treat the iPhone like a cell phone, and not like an iPod. so far so good.