The update “improves the formula to determine how many bars of signal strength to display.”
You can download the update by connecting your iPhone 4 to iTunes and clicking “check for updates”
iOS 4.0.1 is available for iPhone 4, iPhone 3GS and iPhone 3G.
Let us know if you notice any other changes.
Apple has also released a software update for iPad. iOS 3.2.1 software update for iPad appears to be contain bug fixes and improves overall performance.
Download Link for iPad – 3.2.1