Anandtech has a in-depth technical article on the upcoming changes to iOS 4.1 update coming out that deals with the signal issue. Basically, it seems that Apple was being too optimistic with the signal bars in order to make ATT look good. I have a feeling all the carriers pull the same tricks on their phones too, but Apple/ATT are the only ones who got caught doing it.
http://www.anandtech.com/show/3821/i...-41-signal-fix