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TruPhone, a VoIP services provider in the UK, is reportedly developing new software that would enable iPhone users to make calls over WiFi. In turn, this means users would be paying to make calls over the Internet as opposed to over their mobile network, which is generally considerably less expensive. When not in range of a WiFi network, the service would place the calls through the SIM card, as is the case with all iPhone's (and most other mobiles) today.
The software still has a long way to go to be ready for mainstream use, but an early beta is expected in about four weeks. It will be interesting to see how AT&T reacts to such a product, as it could considerably reduce people's mobile usage minutes if the application is successful.