Wojtek Felendzer held a mobile phone to his ear as he walked across the room, the call automatically switching behind the scenes from a Wi-Fi wireless hotspot to the regular cellular network

Check out this article, found via slashdot bring on the Mobile VoIP goodness !

What's really cool about this article as it describes a working implementation...

works by tunneling cellular information packets through the Internet when Wi-Fi is available and reverting to cellular towers when it is not.

This is the first implementation I've seen aimed at consumers, not businesses, no "call manager" needed, just cheap calls !