It seems to effect all iphone 4s
the antennae are not being blocked by a mass of flesh and bone (head/hand) - it's just a small electrically conductive object that is connecting the two antennae segments together. And when you have a sweaty hand holding the phone in that area I'm pretty sure it will do a fairly passable impression of an electrical conductor.
The head and hands do block the radio signals on all phones, this is true, however the iPhone 4 is the only one designed with the metal aerials on the outside. The bluster from Apple about hands blocking the signal is pure smoke screen - it's not the problem, the design is the problem.
However some lacquer on the exterior of the antennae would solve the problem - nail varnish would also do the job for those already with an iPhone 4 - in which case this problem should be addressed as a manufacturing flaw/defect.
Just look at this youtube video where this guy demonstratives this issue with a key:
http://www.youtube.com/watch?v=zvH-i7rKPJc
This guy reckons he has tried it on half a dosen handsets and they all have this problem........