I never understood the need. Death grip or not, my iPhone 4 doesn't drop calls, doesn't lose signal when in an area with decent coverage, etc. I don't know anyone here that has such issues.
I'm on the Rogers network in Canada.
A buddy of mine asked about where to get a good bumper for his iPhone 4. I thought he was crazy. Then I realized he bought into all the anti-iPhone 4 baloney that was spread some months ago. Nice to see that reality (the reality of sales, that is) has cut through all the crap. He did what I did: bought a very nice Sena leather "Ultraslim" pouch and he's good to go.
All in all, I just feel sorry for users that have to live with US carriers.
I can easily make my iPhone 4 go from full signal + 3G, to 'No Service' by holding it with my skin touching the part on the antenna.
I tried a case but found it not to my liking, in the end I just developed a new way of holding it, so that it is untouched on the bottom left black line, which now just feels natural.
I know a lot of people just jumped on the Apple hate wagon, and bitched despite not even owning one. But that doesn't mean the problem is very real. Of the 9 people I know with iPhone 4's, all can replicate the problem with a single finger.
I guess it's now all forgotten about because Apple seems to be able to get away with more than other companies, and has a much more loyal fan base than most.