Another possible cause - some sets were not coated ? Another sign of poor quality control : excerpts from http://www.electronista.com/articles/10/07/02/claims.maintain.apple.had.bad.production.run/#ixzz0sjbQbWqG
A tentative rumor largely gone unnoticed has raised the possibility that the iPhone 4's reception problems may have been caused by production issues. At least this and one other report claims that Apple support representatives were told there was an issue with a coating meant to prevent accidentally bridging the antenna but which wasn't present on all units.
While room for doubt exists, the rumors would potentially explain the inconsistent behavior of the glitch. Some customers have had few or no issues regardless of where they use the phone, while others have had signal problems even in known good areas.