I can consistently reproduce the signal loss at home and at work. And in both places the bumper alleviates the problem.
Yeah I can't do it at work because the tower is just right there. 1.7 miles away though at my home I can do it repeatedly.
I don't have the bumper yet as I ordered it when I ordered my iPhone. However, I decided to go ahead and get a case at Best Buy today. In my normal daily routine I can say without a doubt that the case has put my iPhone 4 on par with the 3GS I previously had. In some areas I was even showing more bars than my co-workers with 3G's and 3GS's.
I personally think ALL these iPhone 4 have this issue, it is just that some have simply not been away from a very strong signal for it to show itself, but it will. Most people use a case so for many this is a non issue.