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Yes, just tried for 15 minutes and couldn't replicate the issue.

Engadget also claims that the bumper solves the issue and I think they're right
 
Not using a Bumper, per se, but I picked up a Griffin Reveal, which has a similar rubber border, and it does seem to eliminate, or at least lessen, the issue.
 
Phone on table by itself (no bumper) = full bars
Phone in hand during call (no bumper) = 1 bar

Phone on table by itself (bumper) = full bars
Phone in hand during call (bumper) = 1 bar

conclusion: didn't do a thing for me.
 
The bumper hasn't completely eliminated the reception issue. At least when attempting to use the iphone 4 inside my house where I don't get greatest AT&T reception. I typically get 2-3 bars, occasionally going up to 4 bars. Without the bumper, I can consistently get the phone to lose reception completely with left-handed death grip. With the bumper I bought this morning, the phone still loses a couple bars, down to 1 bar, still allowing me to make calls. However if I go outside and walk down the block where the AT&T signal is 5 full bars, I don't lose any bars with the bumper. Without the bumper, I lose a couple bars.
 
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