I've got a fun bit to add to this discussion - after using my fake Sector 5 for several weeks, I noticed a huge degradation in my WiFi performance - as in, I couldn't connect to any router that was any more than 10-15 feet away. Since I'd had problems with previous metal-bodied cases, I immediately tried removing the case. No improvement, not even miniscule, with a wifi signal meter up on screen.
I tried restoring my iPhone which killed my jailbreak. Just in case. No improvement.
Finally I was about to go in to Apple to see if they would replace my phone which has become almost unusable due to the poor reception, mostly on Wifi, but also seemingly not very good on cellular either.
So I peeled off the carbon fiber back sticker that came with my fake Sector 5 case. INSTANTLY my wifi meter goes from zero to FULL signal strength. Cellular reception is about 1 bar better in the same location.
After having the case for awhile I got tired of the raggedy felt or fake suede or whatever material was on the back with the Element logo, and replaced it with the carbon fiber sticker just to protect the phone in the meantime. Not my personal favorite look, but I didn't want to scratch my phone up, so hey, why not. I really thought it was a fake CF print or something, but it appears to be real carbon weave (try folding the sticker and it'll snap, the material seems legit) and apparently it's conductive enough and appropriately oriented to act as an RF shield when covering the back of the phone. Everything is 100% better with that sticker pulled off. Even battery life is HUGELY better because it's not always fighting to hang onto a signal.
Long story short - DON'T USE THE CARBON FIBER BACK STICKER!