Non-bumper cases, specifically ones that slide on to the phone, significantly increase the danger of trapped particles scratching the back of the phone while taking the case on and off. Case-caused scratches may be annoying, but they are nothing new. This particular problem, however, results in the scratches leading to cracking and/or shattering of the whole back panel. In other words, if you put a slider case on your iPhone, there's a possibility the next time you pull the phone out of the case, the glass will be broken because of this design flaw.
As a result, Apple has severely limited its reselling efforts of third party cases from the Apple Store, even ones that are certified as "Made for iPhone." This really hurts the manufacturers who designed and built slide-on cases, and who pay Apple a cut to get the certification badge so they can be sold in Apple Stores. None of the third party cases still on Apple's website and in its stores are the slide-on ones.
It's not clear how widespread this problem is for those with slide-on cases, but we doubt Apple is going to comment unless the problem gets a lot of coverage. The company likely won't offer fixes or workarounds either. We can expect a completely different form factor for the iPhone 5 though.