Anyone who suggests that the problems are caused by not keeping the phone in a case is a tool. This is supposed to be a finished product, not a breadboard or an electronics kit for which you must buy your own case. And it's a PORTABLE PHONE, no less, which is supposed to withstand the rigors of being carried around. In ANY pocket.
If you think this thing requires a case, why on earth did you buy it? Have you no self respect? People who don't stand up for themselves and DEMAND HIGH QUALITY are to blame for the continual decline of quality in everything we buy.
With the exception of the inexcusable and embarrassing headphone jack, the physical quality of the first-gen iPhone is outstanding. The choice of glass for the screen eliminated a primary problem with non-clamshell phones: scratched-up screens. Don't put film on your iPhone screen! It's glass for a reason!
Like the headphone jack, this cracking issue looks like the result of poor or nonexistent testing. It's hard to imagine how either of these problems made it out the door.