I keep mine in a case;therefore,i haven't had any crackes/scratches on the back but if you use it naked then it can scratch easily and will easily break if you drop it few times,that's because of its glass.
Its not a serious issue though IMO,many people keep their phones with a case.
I guess then, it is a matter of ensuring the phone is clean (and the case) before putting a case on. It seems to me though that smart/cell phones in general should be to a point where scratches from everyday use should be eliminated. Then again, I'm not a polymer expert;)
So far so good for my iPhone4 with no cracks. I did have to get my iPhone3 replaced for what they (Apple) called a pressure crack in the back. They said it happens to users who have the phone in a tight case and keep it in their pants pockets. It happened once and so far it has held up nicely as my 6 yr old's "large iTouch"
Does anyone use screen protectors? I mean seriously?
I have back and front screen protectors. If you don't have them, get the $2 back and front kit from monoprice. They are the best. I have tried a few on ebay and some more luxurious $5 ones but most of them either had poor cuts or the oily rainbow on the plastic.