
There is a technology available called UMA (Unified Mobile Access) which, with the right handsets, will seamlessly switch from cellular to VOIP over WiFi. It is what is used by the T-Mobile "Hotspot @ Home" plans in the US. The phones have cell and WiFi, and if you are at home or on another WiFi connections your calls (in and out) are VOIP and don't count against your cell minutes if you pay the $10 or $20/month. I use it because our building at work seems to block GSM cell signals, so I use WiFi to get signal at work. I also use it at home where the cell signal is a bit iffy.
The iPhone seems to have all the hardware required, but you can bet your life that AT&T won't be offering this any time soon, because their customers might actually like it...
You will never amount to much. -- Munich Schoolmaster, to Albert Einstein, age 10