I went almost a year without a case or screen protector on my iPhone 4 and it's still in great condition. However, I took good care of it and didn't drop it. One above-waist drop, and you're most likely done. The back and front is glass, and cracks easily. Very easily.
I like cases, it makes the phone feel more durable and easier to hold. It also separates yours from others.
I put a Otterbox Defender on it and am so glad that I did so. My girlfriend knocked it out of my hand at a baseball game while walking down to our seats, and it went down a whole flight of stone steps. Not a scratch on either the phone or the case. I'm generally careful with my technology (especially my iPhone), but sometimes, it's better to be safe than sorry.
i changed my mind and got an otterbox defender real tree. the case looks cool and i love the girp that the case gives me. the only thing i dont like is the head phone jack is covered,
You can find the wraps here: http://theluckylabs.com/iphone-1.html They're GREAT!! I highly recommend them, precisely cut and very durable. I found the Apple bumper on eBay, it was $12, MUCH better than $30 at the Apple store...and it was a genuine Apple bumper, came in the original packaging.
I use it with a screen protector, that's all. As long as you don't drop it too hard (my phone can handle a lot I guess, because I've dropped it a lot)