GSM to WiFi handoff

A colleague of mine is working in a project in Seattle for Alcatel on the Spatial Switch . It is being deployed by T-Mobile for a service called UMA, which allows a GSM phone to make calls over wi-fi. Not your GSM phone, of course you have to buy an GSM/UMA phone. But isn't that cool? It will actually handoff a call from GSM, as you walk into your Wi-Fi network range, to the Wi-Fi without dropping the call. A perfect union of GSM, Wi-Fi, and VoIP.