If you take your phone back to the apple store and they admit the problem and their suggestion is to get the bumper then ask them is that their remedy?
Then tell them the phone is under warranty and why should the customer pay for the remedy? The law is that you are entitle to repair, replacement or refund, none of which should cost you any money.
If they tell me to hold it differently, like out of my natural holding position, I will probably sound like an ass but i might tell them that if i get some orthopaedic injury as a result of holding the phone out of my normal natural position from their official advice on using their phone then i will be suing them for personal injury.
I have nothing against the phone (most products have bugs, albeit never seen one this major), but their replies are what i am angry with most. Do they think we are idiots?