Well, this seems the most thorough (and well written) breakdown of this iPhone 4 reception discussion.
My tuppence worth is that I seem to get a loss of bars on 3G, sometimes, when I grip the thing tight, wrapped in a bumper or not. I replicated this exactly on a colleagues iPhone 3G however. Both on O2. So, er….
Meanwhile EDGE is not affected anywhere, at all, regardless of grip etc.
Bottom line for me - the thing is a joy, zippy fast, the camera is great, I’ve not lost a signal or dropped a call and even on one bar, the actual download time doesn’t seem impaired.