The cases in question are the sort that the phone slides into, as they feature a backplate of their own. Presumably the fit isn't quite tight enough to stop dust/dirt getting inside and causing scratches, which may even end up actually damaging the iPhone by cracking it.
This news may be particularly relevant to people who can apply for a free iPhone case (an offer by Apple because of the squeeze-of-death issue). Almost all the free cases on offer have a backplate; Apple's own product is a simple rubber bumper that only wraps around the sides of the phone, sits extremely snugly and should be much less prone to trapped dirt issues. I would strongly recommend this as the protection of choice.
Read more at NetworkWorld.com: Apple reportedly taking steps to head off iPhone "glassgate" and at gdgt.com: With Antennagate over, is Glassgate next for the iPhone 4?.