I've been keeping up with the whole iPhone 4 antenna problems and read about the rubber/silicon bumpers improving the signal. I got a old iPhone(edge) silicon skin lying around so I did a experiment to see if I would get a better signal with the skin on my Incredible.
Sitting on Bed: 83-88dBm
Holding in hand without silicon skin: 95-100dBm
Holding in hand with silicon skin: 90-91dBm
It seems there is a little improvement in signal strength.
Just thought I'd share....