T-Mobile (US) trials wi-fi to cellular hand-over for calls
Other operators will watching T-Mobile's trial of its HotSpot @Home service in Seattle closely. The service permits users to make VoIP calls from their mobile handsets (wi-fi enabled of course - T-Mobile hotspot preferred!) and switch seamlessly over to the cellular network when the user drops out of wi-fi signal.
T-Mobile also provide a wi-fi router for free - so they must be taking this seriously! Some reviews at RCR News.