Some people are having pro lems with call reception due to the positioning of the antenna.
This can be said of most phones, as keith said- we were discussing this on friday, and between us, we have an iphone 3G (poor me!), two 3GS's and one lucky sausage with a 4G. We all tried to 'kill our signal'
Take off any cases etc that you may have on your phone, and grip it tightly in your hand making sure you're touching the metal bit in at least two different places. All of us could get the signal to drop to basically nothing!! But yet even with the 'no bars' reception, we could all call each other. Weird.
Yes, I have too much time on my hands. They'd just announced this and we were waiting for a couple of others to get to a mates house before we all stayed in and watch a so totally not stolen copy of The A Team. FOOL.
This may have been a problem with apple for a while, but with so many 3G/S users having cases on their phones, you've probably not noticed. With the 4G apple started telling people you don't NEED the case, so more people will have noticed. But the problem may have been around for a while, and I'm sure this isn't iPhone specific, either.