Users of the Apple iPad (v1 and v2), iPhone (3GS, 4) and iPod Touch (3rd generation) may start to download and install the latest iOS 5 firmware.
According to Apple, the update should bring more than 200 new features to existing devices, including improvements to features such as photos, newsstand, safari and camera, as well as completely new features such as Notification Center, Reminders, iMessage and PC Free.
In addition, iOS 5 sees the introduction of wireless updates and iTunes syncing, tighter Twitter integration, Airplay mirroring as well as the debut of Apple's cloud computing initiative dubbed iCloud which will backup music, photos, documents and other files on the cloud.
Perhaps the most important improvement though is how iOS 5 will give a new lease of life to existing devices.
An initial investigation carried out by tech news website Anandtech estimates that the iPhone 3GS with iOS 5 will perform just as fast as the iPhone 4 with iOS 4.3, while an updated iPhone 4 will be significantly faster after the upgrade.
Unfortunately, the most exciting new feature of the iPhone 4S, Siri, won't be available even after the iOS 5 upgrade, because Siri is reliant on hardware features present only in Apple's A5 SoC.