Blocking or bridging the embedded antenna on any cell phone will produce the same results. Apple is high profile, so they got the business, so to speak. Materials that do not conduct electricity like wood, drywall, plastics, and glass will impede a cellular signal, but not block it.
But I really think the problem had to do with bridging that space in the antenna with a conductive material, such as the skin on fingers. The case merely provided a few mm of room for the signal to be able to squeeze through, plus it insulated conductive skin to prevent electrical bridging and deattenuation of the antenna.
Having owned an iPhone 4, I personally never experienced the problem beyond the same exact issue I experienced with a Motorola v551, which is that when placed on a table untouched, the signal strength increased, but then touching or holding it, the signal strength decreased. This can be reliably reproduced over and over again with any cell phone in an area of weak signal. Something about the conductivity of human skin interferes with attenuation of embedded antennas, and this has been true from the first cell phones with embedded antennas and is true of all modern cell phones, that in an area of weak cell signal, any skin contact will reduce signal strength and show one or more fewer bars of signal strength until skin contact is removed.
Apple conceded to a flaw in the design and settled a class action lawsuit, but apparently a few are still needy enough to require Apple be punished forever. The complainers had nothing to compare it to, so they were all, all of them, merely mistaken, the flaw exists in all cell phones with embedded antenna. Instead of proving them all wrong, which would have been academic, Apple laid down. What more would you like them to do?
