Visited the Apple Store in Shibuya yesterday. A clerk told me the lightning cable opening is a "weak point" in the case design, and said he would get permission from a manager to exchange the case under warranty.
The manager then told me that the case was "too old" to qualify for a replacement. The case is 8 months old, and the warranty is supposed to be for 12 months.
I didn't feel like arguing, so I simply said fine and left the store. It was a disappointing experience, though. Had I abused the case, I could see them refusing the exchange. But other than the crack at the cable opening, the case looks brand new. The crack certainly wasn't caused by abuse.