Apple has released a new update to iOS 4 with a minor revision of iOS 4.0.1, the new update for the iPhone 3GS, iPhone 3G, iPhone 4 and iPod Touch tweaks the signal indicator which displays the signal strength.
The iOS 4.0.1 update is not the same as the iOS 4.1 beta released yesterday and does not improve the signal strength but only changes the way the signal bars are displayed. According to Apple, the algorithm they had earlier displayed additional bars which is why people had seen the drop in signal bars.
However, this just looks like a cover-up and may just resolve the display issue and not the actual call dropping aka Death Grip issue which has been plaguing a lot of users. To upgrade to iOS 4.0.1, plug in your phone or iPod touch to your PC and click on the "Check for Updates" button in iTunes. Did you update to iOS 4.0.1 and see the call drop issue vanishing or did it just fix the display?
Note: If you have jailbroken your iPhone on iOS4, do not update to iOS 4.0.1 yet as it will render the iOS 4 jailbreak useless, also there is really no reason to update to iOS 4.0.1 as it just fixes the signal bar rendering and nothing else.