There has been a lot of talks about the antenna and what happens if you touch it, but it is not always that you have to touch it to cause weakening of RF signal. Just having something (finger) in very close proximity will also affect the antenna and its tuning.

Human flesh/finger is somewhat electrically conductive and does change the antenna tuning when placed in close proximity to the antenna. Depending on the exact spot in the antenna it can either amplify (yes... possible...) or weaken (most cases) the received signal. Even if the antenna was completely covered with a thin insulating coating, the capacitive connection between antenna and finger will change the antenna tuning with these frequencies. Phones with internal antenna you can pretty easily guarantee at least ~2mm physical distance and minimize the proximity effect, but with this new iPhone antenna that is at the surface only a "thick bumber" will accomplish what other "normal" phones do have by nature.

In new iPhone pictures we have seen the small gap between the two different antennas. Shorting that point with a finger seem to have the most effect on signal strenght. Effectively placing finger over that spot either do add length to the physical antenna and detune it to wrong frequency, or ground the end... in either case the antenna gain will degrade significantly.

If a finger was placed over the gap with a thin insulator between the RF signal still "leaks" to the finger and to the other antenna due to capacitive connection. (Think about how the capacitive touchscreen works...) While this is not as bad as direct contact it is still significant.

I'm still amazed how they let this kind of antenna design pass through into final product? They were either clueless, or did know something that no other RF design houses know. (If so, they must have filed a patent already?)

Just my 2cents... I'm not RF design engineer but do work closely with designers that deal with this kind of stuff. Sometimes it is pure magic what a seemingly small physical change (board layout or component placement) does. At all cost you would like to avoid any other variables (finger) to get into the mix and basically undo all the tweaking you have spent weeks doing the fine tuning...