I have an engineering background and while I have by no means conducted a scientific study, I can tell you with utmost certainty that the iPhone4 does hardly has a signal reception problem.
It has a signal TRANSMISSION problem.
I've done quite a lot of mucking around with speed test apps on the device and I have observed that while touching the left gap between antennas does cause a marginal decrease in download speed, the signal is by no means blocked. To me, it appears the device can still hear the signal from the tower pretty well even with a human hand to detune the antenna.
Upload speeds, on the other hand, are severely crippled or blocked altogether. It appears that touching the gap has an extremely negative impact on the device's ability to emit a signal strong enough for the tower to hear. This theory is supported by call tests I conducted in which the other party was unable to hear me whenever the gap was touched, even though I could hear their voice just fine.
Quite by accident, I also happened to set my phone down next to a set of computer speakers which were very sensitive to cell phone radio interference, resulting in the typical "GSM Buzz" which most of you of you have surely experienced. What I discovered was that a single fingertip over the gap would almost completely eliminate the speaker buzz due to the interference. Touching anywhere else on the device had no discernible effect. Once again, it would appear that touching the gap severely hinders outgoing transmissions from the device, even over extremely short distances.
As I said, these observations are about as un-scientific as it gets, so feel free to draw your own conclusions. But as far as I can tell, touching the gap is enough to stop your phone's outgoing signal from ever reaching the tower, and a tower which thinks your phone is no longer there isn't going to maintain your call for very long.