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As has been discussed, every iPhone 4 has the signal attenuation issue and operating a "naked" iPhone in a low-signal area will result in dropped calls and slowed data connection.
While it's certainly possible that an iPhone could have an extra defect (effecting signal) that would entitle a replacement, your description of the problem (bumper on, signal. Bumper off, no signal) sounds like the same old issue and I doubt you'll get a replacement.(or that if you did, you'd be happy)
It sucks, but this is a known (and arguably) compensated-for defect.