In this video, an iPhone 4 owner demonstrates how the antenna problem also affects transmission in voice calls. Using only one finger, the voice quality degrades, even dropping completely. He explains the process:

Demonstration of the new iPhone 4 showing problems with TRANSMISSION as well as reception. Note the substantial signal degradation (and subsequent audio dropout) at 2:45-2:50 and 2:58-3:01. By simply MOVING MY FINGER along the side of the iPhone, I can get the call to drop signal and audio!!

The effect his finger has when moved across the seam is undeniable. The problem is real, but the question is, will users encounter the issue using the phone normally, when not stroking their phones? [YouTube]