The iPhone 4's reception issues, which involve losing signal strength when you, um, hold the phone, have been discussed far and wide now. But hey, complaining doesn't fix anything.
This slick Vapor4 bumper, made of aluminum and looking inspired by hot rods or spaceships, will keep you from touching the iPhone 4's wraparound antenna, solving the reception problem. It's also very solid, so it should help protect your phone if you drop it. Really paranoid? Well, you can get a carbon fiber back plate for it as well to really get complete protection.
It ain't cheap, unfortunately. The bumper is $80 and the bumper plus back plate is $100.