Reception problem?...or reporting problem?
It's still early days as I have only had my iPhone 4 for about 24 hours. I too am able to make the reported signal strength dip from 3-4 bars to 1 bar by holding the phone in my left hand (in a normal holding position for a phone you would rather not drop - cupping the problematic bottom corner). However, having said that, I made a few calls using the phone in my left hand and, even on 0-1 bars of signal the call quaility was perfect throughout.
I know its still quite early days but I am begining to suspect that the fancy gubbins that Apple have worked into their phone raido circuits may actually mean that signal dropping to 1 bar is not so much of an issue. In fact, the excelent anandtech review goes some way to point out that the general signal processing is so cutting edge to (possibly) mean that, overall, you still hold more calls than other phones.
I could be way off the mark but I dont hear many considering this argument so wanted to throw it out there. (and no, I do not work for Apple!)