It's already been rumoured that the release of iPhone OS 4.0 later this year will bring the ability to run multiple apps at the same time but now details of how are being exposed by AppleInsider.
AppleInsider writes that iPhone OS 4.0 will allow multiple apps to run at the same time and allow easy switching between them using an Exposé like interface. Reportedly users will press the Home button twice bringing up a "series of icons representing the currently running apps". Once a selection is made the app will zoom into focus.
Also rumoured for release in iPhone OS 4.0 is a unified inbox in Mail and the ability and the ability to add individual contacts to the home screen.