I recently bought Pong's case for iPhone 4 for my entire team at Clean. We all use our iPhone as our primary work line and while we love the phone, we certainly are present to the long term effects of cell phone usage.
What attracted me to Pong was they claim their case "reduces the amount of EMR (electronic magenetic radiation) absorbed into the user's head by between 60% and 95%." Pong says their tests have been performed in labs that are FCC certified. Out of all the solutions out there for phones, they seem to have the most credibility.
Lets see if my team or I notice a difference. We'll report back.
p.s. Our team also uses Earthling Barefoot Mats and we love them. I can totally tell the difference using my laptop with and without the mat. Highly recommended.