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Computerworld - Nearly five months after Apple rolled out iOS 4 in tandem with the iPhone 4 launch, it has released the latest update for the software that powers its touch-screen lineup: iOS 4.2.1 (along with version 4.1 for the latest-generation Apple TV). The free update is a major milestone for Apple: It finally unifies the iPhone, iPad and iPod Touch so that all are running the same operating system with the same major feature set. The update also adds several key new features.

The iPad stands to gain the most improvements -- it's been running the older iOS 3.2.2, which lacks features already available on the iPhone -- but all of Apple's iOS devices that can run the updated software will benefit. However, not all models will be able to use every new feature. iOS 4.2 is compatible with the iPhone 3G, 3GS and 4, as well as the iPad and every generation of the iPod Touch except the first one. Certain features, such as AirPrint, will work only on newer devices. (More about this in a moment.)

Here's a look at the major changes and improvements iOS 4.2 delivers.

The iPad gets the most changes

The arrival of iOS 4.2 will be most significant for iPad users, who have been stuck using an older, more limited version of iOS. Numerous software tweaks and improvements result in a much more polished user experience for iPad owners and will likely spur even faster enterprise adoption.

Threaded e-mail messages, third-party multitasking, fast-app switching, drag-and-drop app organization via folders, the addition of Faces/Places tagging in Photos, and wireless app distribution are all overdue additions.

The iPad already supported major VPN protocols; iOS 4.2 adds support for SSL VPN, giving users another way to connect securely to the office. Business users will also appreciate the newly unified e-mail in-box in the Mail app; you no longer have to navigate back and forth between multiple in-boxes when checking e-mail. And IT staffers will like the additional built-in management capabilities that allow them to better secure iPads in the office.

There are a few changes specific to the iPad. Unlike the iPhone's and iPod Touch's smaller screens, the iPad's larger one allows for the addition of a brightness slider within the multitasking bar, alongside the volume and audio controls. (The multitasking bar is the strip of icons that appears when you double-click the Home button.) Another iPad-specific tweak is the ability to choose photo size settings when you display more information in Mail messages.

And the switch on the iPad's side that has been used to lock the screen rotation becomes a mute switch, like that of iPhone and iPod. The lock-screen option has now been relocated to the multitasking bar, next to the aforementioned audio and brightness controls.