Skype (News - Alert) has just released an important new, update to an iPhone app which enables multitasking.

With this new version of the app, users can receive Skype calls while other apps are running, even when an iPhone (News - Alert) is locked.

And during a call, a user can keep the conversation going while he/she switches to another task, such as checking a movie listing or reading an email, the company said.

When commenting on the new upgrade, TMC (News - Alert) CEO Rich Tehrani said it “should make Skype one of the most-used apps on the iPhone.”

He adds that the most significant aspect to the move by Skype was that you no longer need to pay to make calls over 3G. There was a looming threat that the company was going to start charging after September 1, 2010.

In addition, Tehrani says that this innovation will replace text messaging for a lot of people – that means it can eat into an important source of carrier revenue.

In May, the company upgraded the iPhone app to allow users to make calls over 3G data connections.

Tehrani says Skype needs to be very careful as Apple (News - Alert) is now a competitor with its FaceTime application, which allows seamless videoconferencing over WiFi networks.

In fact, Skype has to be worried about losing its lock on customers, he said.

In addition, MSNBC columnist Wilson Rothman said the upgraded app opens the door for other VOIP providers to innovate on the iPhone, such as Google Talk or Line2 (News - Alert).

Skype is counting on the upgrade leading to more calls.

“At Skype, we believe that better call quality and better availability (which is achieved with an app capable of multitasking and/or making calls over 3G) lead to increased call frequency and longer calls,” Skype’s Peter Parkes says in a blog posting. “We also believe that the mobile world is in a period of significant change, for example, with some operators starting to move to tiered pricing models.”

“In light of that, we no longer have plans to charge a supplement to make calls over 3G,” the blog post added. “We’re delighted to make it easier for you to talk for even longer and do even more together using Skype.”

The app works on iOS 4 and an iPhone 4, iPhone 3GS, or a third-generation iPod touch (32GB and 64GB models from late 2009).

The company also updated the app’s graphics to support iPhone 4’s Retina Display.

The app is now available at the iTunes App Store.

Just a few weeks after releasing SkypeKit for Linux, Skype earlier this month announced a new version of the SkypeKit Beta SDK that works on Windows (x86) and Mac OS X.