Apple has released iOS 4.0.1 for the iPhone 3G, iPhone 3GS and iPhone 4. It’s available for download through iTunes. To obtain the update, tether your iPhone to your computer, launch iTunes, click on the iPhone icon and at the Summary tab, click the Update button in the Version field.

The 4.0.1 update “improves the formula to determine how many bars of signal strength to display.”

This is consistent with a promise Apple made in a letter published on July 2, 2010. Thes company said that its testing revealed that iPhones have been misrepresenting the number of bars to represent signal strength all along.

“Our formula, in many instances, mistakenly displays 2 more bars than it should for a given signal strength. For example, 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,” reads part of the statement from Apple.

Apple indicated this update would use a “recently recommended formula” from AT&T to more accurately display signal strength. What’s more, it makes “bars 1, 2 and 3 a bit taller so they will be easier to see.”

Earlier this week, Consumer Reports rated the iPhone 4 higher than any other smartphone in its tests, but stopped short of recommending it because of a problem with signal reception. Consumer Reports’ blog disputes Apple’s suggestion that the problem is related to signal strength calculation.

Apple plans to hold a special press conference on Friday to discuss the iPhone 4.