Apple iOS 4 is released for update and even iPhone 4 is now available in Apple Store. There are some very minute changes in different sections of iPhone iOS 4 and the previous version. Changes appears in Safari, Mail, Data Settings, Call window, Home screen, iPod etc.
Here are some comparison pictures taken by nikf where left side snap is of old iPhone OS and the right side picture is from iPhone 3G running on iOS 4.
Comparison of Small Changes in iOS 4 and Previous OS
Calling Window
Safari Browser
Home screen
Data Network Settings
For detailed comparison of 16 subtle changes in iOS 4, read this article and the other one here.