VoIP on the iPhone has been around for a while - albeit in an unofficial manner that requires a "jailbroken" handset. But a company called Global IP Solutions (GIPS) is now making the feature available to iPhone developers for licensing, enabling them to integrate real-time VoIP technology to games, social-networking apps, instant messenger clients, and of course applications for making calls to regular phone lines over the Internet.
The technology in question is called VoiceEngine Mobile and the company behind it is also responsible for the popular iLBC codec standard (approved by IETF in late 2004) which optimizes calls made over the Internet and is currently implemented in the iPhone. Due to Apple and AT&T's restrictions on VoIP over 3G, however, the feature will only work with the iPhone's Wi-Fi connection for now.