Apple has unveiled its iPhone 4, the thinnest version of its touchscreen smartphone so far.

The device, launched yesterday at Apple’s Worldwide Developer Conference in San Francisco, features video-calling functionality and multitasking among other enhancements.

“For 2010 we’re going to take the biggest leap since the original iPhone,” Apple CEO Steve Jobs told the conference.

“Stop me if you’ve already seen this,” he joked when showing off the new design, in reference to the recent incident when a prototype of the device was discovered after being left in a bar.

Apple said the screen on the iPhone 4 is its highest-resolution iPhone screen ever, with four times as many pixels as its predecessor, the iPhone 3G S.

The smartphone is also a quarter thinner than the iPhone 3G S and uses Apple’s homegrown A4 processor.

The new handset features two cameras, one on the front above the display and one on the back next to the LED flash. Alongside the fourth-generation device, Apple debuted a new video-calling application called FaceTime, which will use the front-facing camera to enable users to see and be seen by the person they’re talking to.

The handset will also run iOS 4, the latest version of the iPhone operating system. Using iOS 4, the fourth-generation device will be able to multitask, which Apple said will allow users to run third-party apps in the background without slowing down the performance of the main app.

This means users could receive VoIP calls while other apps are running, or when the iPhone is locked. It also means apps that use GPS can continue to run in the background while other apps are employed – for example, a GPS app could still provide turn-by-turn navigation while a user listens to music through iTunes.

The iPhone 4 will be available in the UK from 24 June and operator O2 has already said it will be offering the device, although it has yet to provide tariff information.