Apparently, according to a BBC 5Live interview today, iPhone 4 users who go to the Apple store to complain about the 'left hand lost signal phenomenon' are being given a free Apple Bumper to alleviate the problem.
Post on this thread if you have success in getting one.
I did wonder when first seeing the bumper exactly why it had been designed to only protect the edge. And, AFAIK, it's the first time Apple have offered a 'case' for sale with a device.
This suggests that Apple was pre-empting a known problem.