Apple is apparently investigating some 3rd party cases that have been reported to cause more damage than good. Apparently, certain hard, slide on cases can trap dirt in between the case and the iPhone. This dirt can create hairline cracks on the iPhone, which, over time, can lead to the glass cracking.
An all glass back sure looks good, but it certainly isn’t as tough as past solutions for the iPhone. But the right case can compliment the iPhone’s style. Or, you could go all natural and take the risk. Whatever you do, avoid cheaper cases that don’t fit well, and hard slide on cases.