Apple on Thursday gave a sneak peek at a developer preview of iPhone OS 4.0, the next major version of its operating system for mobile devices.

Apple CEO Steve Jobs boasted over 1500 new application programming interfaces (APIs) for developers in iPhone 4.0, letting them access features like the iPhone’s calendar, photo library, video-camera data, and more. App creators can also take advantage of OS X features like Quick Look, in-app SMS, and more. Plus, there are over 2000 APIs for hardware-accelerated math functions.

In addition, iPhone 4.0 boasts more than 100 new user features, such as playlist creation, 5x digital zoom in the camera app, tap-to-focus for video, auto photo-geotagging, the addition of Places functionality, the ability to change the Home screen wallpaper, improved spell-checking, and support for Bluetooth keyboards. Many of the features added are already available on the iPad, which currently runs iPhone OS 3.2.

Apple iPhone

Instead of running down the entire extensive list, Jobs focused on seven “tentpole” features that will be part of iPhone 4.0.

Multitasking

Easily one of the most anticipated additions to the OS, multitasking will be a part of the iPhone 4.0 update. The feature is already offered on rival mobile platforms—most notably, Google’s Android OS—but Jobs believes Apple has done a better job implementing multitasking capabilities.

“We weren’t the first to this party, but we’re going to be the best, just like [with] copy-and-pate,” said Jobs, alluding to the feature introduced in last year’s iPhone 3.0 update.

In order to switch applications, users double-click the Home button which summons a dock-like strip showing all running applications; tap on an app to switch to it. If you’re playing a game, it will pause when you switch, and you’ll be able to resume right where you left off when you switch back.

By default, that dock shows you four running applications, but if you have more running at one time, you can swipe left to see the rest of them.

Apple contends that multitasking on most mobile devices can hamper performance of the app running in the foreground while draining battery life. “We’ve figured out how to do it and avoid those things,” Jobs said.

Specifically, Apple looked at existing iPhone applications in the App Store and tried to identify what services are needed in the background, according to senior vice president of iPhone software Scott Forstall. Apple then wrote its own code to implement those services, which it has made available to third-party developers as APIs. That way, the system is responsible for maintaining battery life and performance, rather than having each additional app try and figure it out.