I do not own an iPhone 4. However, I used one at the Apple Store. it had full bars, and I called my cell phone (iPhone 3GS it also had full bars too) Then I held the iPhone 4 like I do with my 3GS, and watched th bars go down and the call dropped. I tried it, again, but using my iPhone 3GS. It didn't drop any bars OR the call. Again. Same location, iPone 4 kills the call. I used a Droid X at the same mall today. It had full bars. I held it, and tried to drop the call. No bars lost, no call dropped.
I should point out, that my Apple Store has an AT&T repeater in the store, yet I managed to drop the call. I'm sure there was a Verizon repeater in the mall though.