I had an odd problem with my iPhone 4 (which I've had since launch day). At first I noticed a bright spot on the screen, and over a few weeks the top right corner of the back cover started to separate from the rest of the case. I figured the battery was starting to expand, but I was really surprised since that usually doesn't happen in just a few months. But when I brought it into the Apple Store and the phone was disassembled, the Genius confirmed that the battery did expand. The phone was replaced without issue, but I am still a bit surprised. In any case, if anyone starts to notice a bright spot on the screen or a sudden gap in the case, the battery may be the cause.