Apple is now accepting developer builds of apps that've been updated for the newly dubbed, "iOS 4" mobile OS, which introduces many new features for developers and users that may require modifications to existing apps for full compatibility. Some of the features include: Multitasking; Folders; Improved Mail App; Enhanced Enterprise support; iAds; iBooks.
Apple claims there're over 1,500 new Application Programming Interfaces (APIs) available for developers. iOS 4 will be available to iPhone and iPod Touch owners for free on June 21st. iOS willn't run on original iPhone or 1st generation iPod Touch and will only run with limited features (no multitasking) on the iPhone 3G and 2nd generation iPod Touch.