I've only been able to reproduce it in areas where the signal was awful to begin with. In areas where my 3gs drops calls weekly, using other people's 4's I can reproduce a dropped call with some regularity. My guess is that it's so hard to reproduce because even by attenuating the signal by holding it wrong, a lot of users are in an area where the signal is strong enough that it doesn't matter.
Anandtech had an interesting article where they showed both the 3GS and 4g having problems based on how you hold it. With the 4g's worst case being better than the 3GS's worst case, however the loss was more dramatic so the perception is worse even though the device is significantly improved.
I've heard enough "cannot reproduce" reports to suspect it's a minority of devices, still that's a lot of people affected. My suspicion is that there is a coating process that is failing in some cases, which would mean it could probably be resolved for future production runs.