There have been rumors of video-calling coming to mobile versions of Skype for the last week or so, and now it's official: the popular VOIP service has announced an update to its iPhone app that will allow video calls over both 3G and WiFi networks.

The new Skype app works on iPhone 4 and 3GS and the iPod Touch with iOS 4.0 or above. You can also receive video calls on 3rd generation iPod Touch devices and on the iPad. Video-calling was already available on Skype desktop clients.

The new Skype app expands the number of those who have access to video-calling on their Apple devices, as Apple's new Facetime only works on iPhone 4 and Mac computers.

As the response to Skype's recent outage demonstrated, the service has become an important communication tool - for both professional and personal reasons. Skype says there are approximately 25 million concurrent users logged into Skype at any given time.

And video-calling makes up more and more of that usage - approximately 40% of all Skype-to-Skype minutes for the first six months of 2010, according to Neil Stevens, Skype's general manager of consumer business. "By bringing video to mainstream users at their home or work via their desktops, on the go with their mobiles, or into their living room via their TV, Skype has made it possible for millions of people to share their video moments wherever they are."

"Wherever they are" is increasingly mobile, and improvements to Skype's iPhone app are welcome news. No word, however, on when video-calling will come to Android.