I was in a classroom and I was playing around with the iPhone - definitely noticed the lowering of signal when I was surfing the net in my left hand. But now - here I am at home - trying to reproduce it and I can't.
I think that all iPhone 4s have the issue, but it is mainly dependent on the service in a particular location. I'll keep testing it at different places to see if I notice any differences...
Overall though - this is one cool little device, no?