This has been accomplished by TruPhone, a softphone VoIP solutions provider based in the UK.
Not even ready for beta, what I have just referenced is a rather pre-alpha cross-your-fingers-and-hope-it-works setup that, Starr writes in part:
Currently requires the use of terminal on the iPhone to tell the iPhone to use its on-board SIP stack to place the call over WiFi instead of via the SIM card. To use the terminal application, in turn requires that you first Jailbreak the phone using an application like iBrickr or iFuntastic. This is not an application for the inexperienced or the faint of heart.Instantitating the SIP Stack for TruPhone.
Still, Oliver emphasizes that, well, it's kinda early. TruPhone says it will simplify the application and will add seamless switching between VoIP and open WiFi systems, with the SIM card kicking in when the iPhone is out of open WiFi range.
Update: Andy's just posted a video of the TruPhone at work on the iPhone. The video is here.
Like JGeils (I know I am dating myself there) , "freeze frame:"
I agree with Oliver that the real plus-plus will mean an alternative to international roaming fees charged to iPhone users.
ETA to beta is about four weeks, but because TruPhone wants to ensure capacity, this will be one of those invitation-only, limited betas. So if you are tempted, I would advise waiting until enough information is gathered from these early beta testers.