I got my ip4 at launch and a few days later the bumper for protection.
I was getting a network lost message popping up which I had never seen on the 3G. I could affect the bars by holding it in my left hand both WITH and without the bumper on.
Only when I compared to others side by side did I realise that mine was not right. So a genius bar appointment was arranged and the genius agreed it was not right so I got a new ip4 and all has been well since.
And.... I just noticed that apple have refunded my bumper purchase without me even asking! Well done Apple.