Apple says it has sold 450,000 iPads in the U.S. since its debut last Saturday. The new iPhone operating system also allows iPad multi-tasking. (Marcio Jose Sanchez/Associated Press)

The latest version of Apple's iPhone operating system will let users multi-task, company CEO Steve Jobs announced Thursday as he unveiled the much-anticipated upgrade.

The enhancement, which will be available this summer for the iPhone and this fall for the just-released iPad, allows users to run several applications at once.

"We weren't the first to this party, but we're going to be the best," Jobs said to his audience at Apple headquarters in Cupertino, Calif.

The only multi-tasking iPhone users had been able to do was largely limited to Apple's own programs.

With older versions of the operating system, changing to a second application usually meant closing the first one.

But with the operating system upgrade, people will be able to run Skype — the popular voice-over-internet protocol service — and switch to another app or surf the web without losing their Skype call. That would allow people to receive Skype calls — something that was impractical before.

Users will also be able to run a GPS location app in the background while listening to music or surfing the web.

"It really changes the way you use the iPhone. You're bouncing around the apps with tremendous fluidity," Jobs said.

Double-clicking the home button will allow users to see all of the apps running at the bottom of the screen. Tapping on the app you want then brings it up full screen.

Multi-tasking for newest iPhones only

The multi-tasking function will only work with the newest iPhones. People who own the original iPhone or even the iPhone 3G will be out of luck — the upgrade will work only on the 3GS version that came out last summer. The upgrade can also be added to the third-generation iPod Touch models that came out late last year.

Older iPhones will, however, be able to run some of the other new features announced Thursday.

For instance, users will be able to direct multiple email accounts to land in a single box. Or they can connect their iPhone to a regular keyboard using Bluetooth wireless technology.

Apple usually makes such updates available as a free download. As part of the iPhone OS 4 news, Apple also announced a gaming network and an advertising platform called iAd. Apple will sell ads for apps provided by third-party developers. The developers will get 60 per cent of the ad revenue; Apple the other 40 per cent.

IPhone owners have long wished for the kind of full multi-tasking announcement from Apple. Phones from Apple rivals like Palm and Google already allow users of their products to run more than one program at a time.

Jobs said Apple has sold 450,000 iPads since its launch in the U.S. last Saturday. It sold 300,000 units the first day. The Canadian launch is slated for later this month.

The U.S. list price of the iPad tablet starts at $499 US.

With files from The Associated Press