Lisandro and several other readers let us know that Apple has just released a statement addressing the signal issues
a lot of users are having with their iPhone 4. They claim to have discovered the cause for the drop in bars, which is "both simple and surprising" — a wrong formula used to calculate how many bars are displayed for a given signal strength. "Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong. ... we sometimes display 4 bars when we should be displaying as few as 2 bars. Users observing a drop of several bars when they grip their iPhone in a certain way are most likely in an area with very weak signal strength, but they don’t know it because we are erroneously displaying 4 or 5 bars. Their big drop in bars is because their high bars were never real in the first place. ... We will issue a free software update within a few weeks that incorporates the corrected formula. Since this mistake has been present since the original iPhone, this software update will also be available for the iPhone 3GS and iPhone 3G."
Wired notes that there is still a signal drop
when the iPhone 4 is gripped in particular ways.