Apple has just released the anticipated update for IOS 4. The new version is 4.01, which contains bug fixes & improvements, including the formula to determine how many bars of signal strength to display.
It was just seeded to developers yesterday, and it looks like Apple is hot to get this out before their press conference regarding the iPhone 4 tomorrow.
To get your update, connect your iOS 4.0 phone to iTunes and check for an update.
FYI, this does not fix the reception issue. This is a hardware & network issue. Consumer Reports is now advising that the problem is assuaged if you add a 'bumper'.