Video calls are nothing new on smartphones: AT&T has been offering a limited form of video calling through its Video Share service for a while now, the Evo 4G and the iPhone 4 (WiFi-only) have been touting the feature even before they launched, and apps like Fring and Tango have also begun offering video calls. None of these is as big as Yahoo (81 million users), given that it's the second biggest IM network in the world (the first being Windows Live Messenger).
Skype is probably best known for its video calling features, but so far the company has avoided offering the service on mobile phones (though calling over 3G is a go). In other words, Yahoo will be the first mainstream messaging service to offer video calling across a wide range of devices. Expect Microsoft and Google to follow suit.