This makes for interesting reading:
?The metal bezel surrounding the handset would need to be separated in sections to create individual antennas capable of handling particular ranges of the radio frequencies for different wireless networks, the person said. If a user covered one of the seams between the sections, their finger would act as a conductive material, interfering with the signal, the person said. Consumer Reports suggests iPhone 4 users cover the antenna with duct tape to help mitigate reception woes.?
Found the following ( antenna photo part way down)
It looks like it?s a conductivity problem (partial shorting)
I think the ?bumper case?just physically prevents your (partially conducting) fingers from bridging the different antennas present in the case (has to handle 850 Megs, 1800 Megs etc).
A very interesting movie: