Well it looks like multitouch gestures for iPhone 4 and 3GS are present in the current developer-only iOS 4.3 Beta releases. A YouTube video just surfaced shows these gestures in action on an iPhone 3GS, running iOS 4.3 Beta 1.
However Apple has already confirmed that the gestures option for iPad (and probably iPhone too) wont be available to public when iOS 4.3 goes gold. They are probably going to release it as a major feature for iOS 5 when it is introduced with iPhone 5 in June this year.
(Thanks Daniel, Alex)