The iPhone's next operating system (OS) will feature major upgrades, with Apple announcing more than 100 new user features to be available for the iPhone in a few months.

The new OS will be likely be released to the public when the new iPhone hits shelves in June or July, but the OS upgrade will also be available for 3GS iPhone users. Those with an older iPhone will still be able to upgrade, but won't get all the features, meaning 3G users still won't be able to multitask. And because the iPad is built on the same system, Apple said that it will be getting the OS upgrade a few months after the iPhone.

MacGuide previously wrote about the new OS getting multitasking, which is likely the most anticipated feature. Apple CEO Steve Jobs claims that the company has made the OS able to run multiple apps without draining battery life due to the way the company wrote the coding.

According to MacWorld, Jobs said about multitasking, “We weren't the first to this party, but we're going to be the best.

With the new multitasking feature, the iPhone will feature a dock that will show all running applications that users can switch to and from. The 4.0 OS also features folders for apps that significantly helps declutter iPhone homescreens. And those iPhone backgrounds will now have the ability to be customised, just like the iPad.

Throwing in a bit of a curve ball, Apple also said that the iBooks app that is available for iPad, will be incorporated into the new OS, allowing users to read ebooks on their iPhone. The iBooks application will be smaller than that on the iPad, but will feature sync abilities, so if you stop reading on your iPad, you can start at the place you left off with your iPhone.

The 4.0 OS will also include a revamped version of Mail, with a universal inbox that allows you to see messages from multiple accounts. And in order to make the iPhone more enterprise-friendly, the new OS has added features for business users, including encryption for better data protection.

Incorporating a multiplayer game network, the 4.0 OS will feature the Game Center, a social-gaming network that has been rolled out to developers and will be available to users later this year.

Among the hundreds of other new features, the new OS features 5x digital zoom and the ability for the iPhone to work with Bluetooth keyboards.

Apple has notoriously been closed in terms of developer access to API (application programming interface), but with this new release the company is also releasing previously closed APIs to developers, such as push notifications, location services and VoIP.

Apple also released iAd, a built-in advertising system for the iPhone that allows developers to incorporate ads into their application.