As you can see in the above screenshot, Apple has delivered on their improved signal bar graphics. Only time will tell if this is an actual indication of signal strength. Other noticeable changes include a revamp of the Game Center app, the ability to add FaceTime favorites and the Camera app (incl. camera roll) auto-rotating in landscape. More screenshots after the break.
The following screenshots were taken off of an iPhone 4 running iOS 4.1 beta..