Little more than a year after we first clapped eyes on iPhone OS 3.0, Steve Jobs is back again with a bigger and better operating system for what remains the world’s favourite Smartphone.

iPhone OS 4 has been announced today with a host of new features. It will be available this summer. Perhaps the biggest talking point is the implementation of multi-tasking. Yes, they’ve finally done it! iPhone owners will now be able to run multiple apps at once, with the ability to ‘double tap’ the home button to switch between running apps. The good news is that Apple says it has achieved this without hindering battery life or performance of apps. The not so good news is that multi-tasking is one of a few features of OS 4 that is not going to be available to those with handsets older than the 3GS. Sorry, folks, but those of you with the iPhone 3G or the original iPhone (or an older iPod Touch) won’t get multitasking.

OS 4 will also give iPhone users spell check, enhanced mail (including the ability to sync to multiple exchange accounts), Bluetooth keyboard support, user-defined wallpaper (something long overdue) and the ability to organise apps in folders.

Perhaps not so surprisingly, iBooks will also be a feature of OS4. This was first unveiled with the iPad but will also be available on iPhone from the summer. There’s also to be a new ‘Game Center,’ which will be something similar to Xbox Live but for iPhone games and will let users share scores etc.

Eager to get your hands on it? iPhone OS 4 will be out in the summer, with the iPad version to follow in the Autumn.