First reports of iPhone VoIP application surface
It's not the first time we've heard the idea (SoonR Talk), but it does look like a true, calls-over-WiFi VoIP client for the iPhone could be just around the corner. Over at Blognation they've had a demo of one such solution from TruPhone.
Whilst the application still requires heavy use of the Terminal (and is strictly alpha-quality), TruPhone is promising a release that "includes simplifying the activation and adding seamless switching back and forth between VoIP when open WiFi is available and the use of the SIM card when out of WiFi range. It is important to note that it is NOT NECESSARY to break the SIM lock to use TruPhone's iPhone VoIP application."
If you've always wanted to use a SIP-compatible VoIP solution on your iPhone, it appears your wish is about to be granted.