Apple iPhone applications getting updated for iOS 4
On the whole, we expect a huge wave of updates bringing larger graphical elements, which can take advantage of the new Apple iPhone 4's 640x960 screen resolution. One of the installed apps, Quota, for example, has already notified the user that it will natively run on the new screen now as well.
Other abilities of the new platform version will surely also be taken advantage of soon. For example, music players and instant messaging clients can now all run nicely in the background, while you are reading that next iteration of Angels&Demons in iBooks. However, they will first have to be updated with the iOS 4 SDK, so they can simultaneously run along with other apps.
source: The iPhone Blog