In an odd twist, cases designed to prevent the iPhone's exterior from scratching actually appear to be causing scratches for some users. A number of posters to Apple's Discussion boards are reporting that the InCase molded rubber sleeve, which is sold through the Apple Store as an authorized accessory, is actually producing blemishes (including scratches) on the iPhone's chrome faceplate border that are visible when the case is removed. Ironically, the same scratches are unlikely to occur on iPhones without cases; when combined with previous reports that some iPhone cases can cause heat problems, it seems an unclothed iPhone may be your best bet.
The problem is most likely to occur with tight-fitting cases (especially rubberized ones like the InCase), and appears to be the result of dirt and other particles being trapped between the casing and the chrome surface.
As a possible remedy, you may want try using a jewelry wipe or polishing cloth (such as these, priced at $4) or iCleaner (an iPod polishing/cleaning set) to remove the scratches. Do so at your own risk, however, as these items might cause other cosmetic or component damage to the iPhone. Apple has its own instructions for cleaning the iPhone in a Knowledge Base article.