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 Pandorawhile 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.

When will you upgrade to iOS 4? What feature are you most excited about?