Apple has just released the iOS 4 update for iPhone and iPod touch owners. The iPhone 4 might not be in stores until Thursday, but existing owners can take advantage of the new software features coming to the new device starting now.

iOS 4 is a free update and is compatible with iPhone 3G and iPhone 3GS models, as well as second- and third-generation iPod touches. Multitasking will only work with the iPhone 3GS and third-generation iPod touch, however.

On the backend, iOS 4 has more than 1,500 changes or additions to the API, which will enable developers to build even better apps on these devices. We’ll be posting a more complete video overview of what is new in iOS 4 later this afternoon, but check out some of the highlights first:

  • Multitasking — A combination of persistent-state applications and backgrounding functions for certain services. The end result means better battery performance when doing things like listening to Pandora () while browsing the web.
  • Folders — Organizing your iPhone Home Screen is easier now that you can use folders to store groups of apps in one icon. This is a huge time-saver and makes finding and keeping more apps on your device much easier.
  • iBooks — First appearing on the iPad, iBooks is now available for iPhone users. Just like the Kindle app, your purchases and page marks are synchronized across devices.
  • Tap to Focus Video — iPhone 3GS owners can now tap to focus their videos just like you can when taking still shots. The new software also includes a 5x digital zoom for the built-in camera.
  • Bluetooth Keyboard Support – You can now pair your iPhone with a Bluetooth () keyboard, which will make typing long documents much easier.
  • Better Spell Check – The spell check utility now works across Mail, Notes, Safari () and other apps.

There are tons of other new features in iOS 4 and we’ll be showing them off over the next few days, as well as highlighting some apps that take advantage of the new features.