Boy Genius Report (BGR) guys have claimed to receive some exclusive iPhone OS 4.0 details from one of their reliable sources. Also according to the source, this update will include brand new interface and multitasking feature (which the iPhone users badly missed in the existing and previous OS), and the update will be limited to iPhone 3G and 3GS.
According to BGR, here is a list of iPhone OS 4.0 Features:
1: There will be multi-touch gestures OS-wide.
2: iPhone 4.0 will have Multitasking feature, that is the ability to run applications in background
3: Revamped interface, more easier and efficient navigation through OS due to plenty of graphical and UI changes.
4: There is not much surety for this one but there may be some brand new syncing ability for the contacts and calendar applications.