Yes, almost all smartphones will experience drop in signals but then such phones are designed in a way whereby the antennas are located out of way so that natural grip will not affect obvious drop in reception. Some phones like the Motorola Droid X has got 2 antennas built-in to mitigate such antenna problem.
Apple's latest counter-attack in proving all smartphones are flawed is on Droid X (notice the entire phone vibrates and the position of the index finger)
The difference? For the iPhone 4 to lose signal quality, you only need your palm touching the lower corner of the phone. For the Droid X, you need to cover the entire length of the bottom. It's almost as if, you know, it was engineered for it to be difficult to cut the signal.
I hope Apple stop carrying out such similar demonstrations. iPhone 4 is obviously design flawed. Demo on other phones are mostly either in unnatural grips or super tight grip till the entire phone vibrates!