Handover? (Score 3, Interesting)

So what happens if you move outside the WiFi coverage during a call? Handover between 3G networks and GSM should be possible, but is it possible to switch from WiFi to normal GSM without disconnecting the call? I believe this requires support from the network as well, meaning that the operators will have their say, too. Correct me if I'm wrong here.

Continuing the VoIP traffic over GPRS data could be possible without new features to the network or disconnecting, but that does not sound very tempting, since the rates for standard GPRS are counted in Euros/MB where I live...