Apple showcased its iPhone OS update during Monday’s Worldwide Developers Conference keynote. And while the presentation largely covered the same ground Apple went over when it first unveiled the update’s major features in April, CEO Steve Jobs offered a few new tidbits—a new name for the OS as well as support for video calling features with the latest iPhone.

The OS, which runs on the iPhone, iPod touch, and iPad, has been renamed iOS 4.0—a smart move given the number of mobile devices Apple now offers. The new OS offers over 1500 APIs for developers, and Jobs showed off a number of them on stage.

Jobs revisited a number of the new features that he gave a preview of at Apple’s iPhone OS 4.0 event for press and developers back in April this year. These features include: multitasking, folders for organizing applications, a unified Mail inbox, a new Game Center social network platform, iBooks for iPhone, major enhancements to features for Enterprise businesses, and more. On stage were demos of application switching between Mail and Safari, all the while allowing Pandora, a third-party music streaming and discovery service, to play in the background just as Apple’s own iPod app can.

For enterprise users, Jobs announced that iOS 4.0 would offer better data protection, mobile device management, wireless app distribution, multiple Exchange accounts, Exchange Server 2010, SSL VPN support.

Confirming many rumors from the past few months, Apple has also added Bing as a Safari search option alongside Yahoo. “Microsoft’s done a really nice job on this,” Jobs told keynote attendees. Google remains the default, however, but users can easily change this settings in the Settings app.

Pulling his “one more thing” card, Jobs ended the iOS 4.0 portion of his keynote by demoing video calling on the new iPhone 4. A “FaceTime” button in a phone call allows for users to enable face-to-face video conversation, something Jobs said he grew up “just dreaming about.” FaceTime works between two callers using iPhone 4s (though only for now. Apple intends to open-source the standard so any phone can use it), and you can switch between front and rear-facing cameras in the middle of the call. While iPhone calls are placed over a cellular network, FaceTime will only work if WiFi is present as well to exchange the actual video portion. “This is one of those moments that reminds us why we do what we do," Jobs said.

Apple iPhone 4

Jobs wrapped his iOS 4.0 presentation by announcing that developers can download the Gold Master Candidate build Monday.

iOS 4.0 will be available June 21 as a free update for the iPhone 3G and 3GS, as well as to the second-generation iPod touch; note that the 3G and touch don’t support the multitasking feature. The update will be available for the iPad later this fall.