This doesn't make any sense. Either when you're getting that signal you're holding the phone insanely, or you have a defective unit.
The common signal issue is crossing the line between the two antennas in the lower left. It is not always reproduceable, but a single finger over that area can drop signal.
Now entirely cupping the bottom of the phone might drop the signal... but that's just an effect of covering the antenna.
NO INSANE GRIP!!! Just holding it as delicately as Jobs did in his revealing of the phone and as normal as they do in the FaceTime adds.
Gripping it normally like I did my 3G and 3Gs and had no issues with those.
It sucks cause other then this MAJOR problem the phone is amazing!