Though it's not the first VoIP solution for the iPhone (there are a few Web-based solutions, including one that's Skype-compatible and another from Talkety), Truphone has demonstrated the first -- that we know of -- VoIP solution that is capable of using the iPhone's native phone application to dial calls. It's a third-party application that runs through a terminal interface on the iPhone and "hijacks" the phone application to make calls over a VoIP network.
Andy Abramson has now posted a video of the call being made, embedded below. Unfortunately, if you're on an iPhone, you won't be able to watch the embedded video, but you can search for "Truphone iPhone" in the iPhone's YouTube application and find the same video.
There's no word on when this solution will be released to the public.