Apple has released iOS 4 for iDevices (excluding iPhone 2G and iTouch 1stgen). This is the apple’s most advance mobile mobile OS till date and it provides over 100 new features, some of them are: multitasking, folders, customizable backgrounds for the Home screen, a unified inbox for the Mail application, more fine-grained control over Location Services, support for Apple’s new iAd mobile advertising platform, and support for iBooks 1.1 on the iPhone and iPod touch. Few features like multitasking, background change will only work not work on iPhone 3G and iTouch 2G.
People who are relying on unlock tool like blacksn0w, ultrasn0w should stay away from this update, as it lock their devices. Just wait for sometime until devteam/GeoHot/iH8sn0w/chronicdevteam comes with their jailbreaking and unlock tool.