I too think that UMA makes a hell of a lot more sense than the whole FemtoCell revolution going on. Every smartphone has wifi builtin and with UMA over wifi there is no additional hardware cost to the end user and seamless switching between wifi and towers. I travel a lot and would be able to significantly cut the costs of my phone calls using this technology. If I end up switching to T-mobile I'll probably get a cheap UMA capable phone to use for calls to the US when I am overseas.

As soon as google allows number porting and regular SIP calls I would port my number to GV in an instant and use a poor mans version of UMA. The only thing missing would be seamless handoff between wifi and cellular.