It will be interesting to see with the 5. I have had Element case and e13tron AL and AL hybrid cases for my iPhone 4. The effect on signal strength, while measureable, did not bug me. The cases are beautiful, so this was fine with me until I dumped my Garmin and started using Navigon and the Maps app as my travel aids.
What DID bug me was the effect on my GPS and map apps. I could be driving along and the icon would be a block off! Finally just bailed and went back to plastic. (Minus a couple hundred that I paid for the aluminum cases)