Plastics
Before I rant:
I respect all iPhone 3G users out there. I own a 1st Gen and don't want to rub it in that I don't have cracks. I swear I have a gaping hole where internet speed must be pouring out. It's agonizing loading anything. Like- dial up agonizing. EDGE sucks, but it works.
Anyways,
After owning an iBook, made of plastics apparently used in bulletproof vests, I will never again own an apple product made with that plastic. Regardless of the strength, the plastic (or whatever alien material anyone would like to claim it is), expands when hot, and contracts when cold. For old iBookies, that was worse because so many moving parts in an expanding/contracting mobile case equaled failures and repairs (sure hope those problems are fixed for macbook users). And now as we clearly see in this thread, the cracking has begun.
Now, all material expands when hot and contracts when cold (as men know best), but the aluminum (or titanium, or similar materials) does so the least, providing maximum quality and protection at the cost of added weight. I've held the 3G, and I envy the lightweightedness over my 1st Gen, but still enjoy the rigidity of my phone, because, let me tell you- it has had the crap beat out of it, as cell phones do. For those without cracks, good luck keeping them that way!
It was rumored a while ago that Apple was trying to move to an all aluminum product line after they changed the imac. Guess not. Either way, this is just my assessment of why Metal rules over plastic for quality. Of course, it's heavier and pricier, but better for people like me that refuse cases and still want toys to have some longevity.