According to a growing thread in the Apple Support Discussions forums, the Apple bumper case, made specifically for the iPhone 4 (and a major part of the "Antennagate" solution), is scratching the steel antenna band.
ASD user Dayvidpriddy reports:
"I received my black bumper about a week ago now. Took off the bumper tonight to clean my phone down and there are scratch marks on the steel band from the bumper (this is caused from the pieces of hard plastic on the underside of the bumper."
Many of the comments after the initial post point out that it is extremely unlikely that the hard plastic used to make the bumper would cause scratches on the stainless steel band. What is more likely is that some debris got between the case and the iPhone, causing the scratches.
ASD user richiebarthez, however, reports that Apple acknowledged the possibility that the bumper could have caused the scratching and replaced the unit:
"Apple replaced the handset and admitted the inside of the bumper was very sharp inside. They gave me a new iPhone 4 and new bumper."
I am reminded of when the original iPhone first came out. One of the most popular cases we sold in the Apple Store was the rubber InCase case. Not long after they arrived, reports that they caused scratches and scuffs on the back of the iPhone surfaced and we had to deal with many customer service issues because of it.
With any case purchase, users should be aware of the possibility that debris, case construction, and other factors can result in this type of problem. Of course, with a case designed by Apple for its own product, a certain level of excellence is implied. In any case, there is no harm in taking a moment to consider each product you buy--especially if it is supposed to protect another one.
Now that many iPhone 4 users have received their "antenna-fixing" bumper cases (though I am still waiting on mine), are you experiencing scuffs, scratches, or other marks as a result? Let us know in the comments!