Cracks only happen from user error. Dropping, being rough like setting it down hard or overall just being careless. I'm not trying to push anyone's buttons but I see people handling there phones like foolish teenagers all the time and not realize they are doing it. It's a friggin' $600 (unsubsidized) mini computer and just because you handled you previous crap NOKIA phone from the 90s like a dumb primate and kept those bad habits, don't expect your little hairline cracks to be Apple's fault.
When they happen at the same spot, I'd say there is arguement that there was a flaw in design or plastic. Besides, those cheap phones are made of plastic also, why didn't they crack after being dropped so many times

? Plus, honestly, cellphones also have electronics and stuff that can break. I say this cause I have a macbook that has cracks where the palms are and I heard this arguement for that. Sorry, but no, it was a design flaw. When every single macbook (seriously, everyone I knew with one got those cracks. One guy even got the palm rest replaced and the replacement got hte same cracks) got a crack in the same exact spots, maybe the problem is with the item, not the people.
Personally, I think the iphone is pretty tough for something with a large screen and being a PDA. I'm pretty happy with how tough it is (and yes, mine did get a crack but only after it is almost 2 years old and it works fine and the crack is not on the screen and that's what is important). Yes, I've dropped mine plenty of times. I realize there is only so much you can do particularly with something with a large screen like that and I think the iphone was designed pretty tough. It sure feels more solid than my previous Nokia (which was Nokia's first foray into a flip phone and kinda not done so well. A disappointment for a Nokia). Oh, and just so you know, the crack did not appear after I dropped the phone *shrug*.
But I also think that when you are designing something to be a phone that is going to be handled a lot and picked up and down a lot, it *WILL* get dropped and it *should* be designed to be able to handle some rough treatment (not so much cosmetically but it should still be able to function). If it can't handle a few minor to moderate drops, yes, I do think it's a crappy design. Yes, people should be careful, but sorry, sh** happens, particularly with small objects that are handled a lot and anything designed to be a cellphone should be designed to handle at least a few minor snafus.