but they can't support SIP - a protocol specifically designed to handle these kinds of situations
Can't, don't, or don't yet? With google voice, they're two trivial steps away from letting wifi capable phones call and receive calls anywhere in the US (and probably elsewhere) for free, with no SIM card installed, even - 1) set up a SIP gateway and allow (android/iOS,etc. or computer based) softphones to connect it to it using Google's existing authentication bindery, 2) allow Google Voice users to direct calls to the gateway (and 3), I guess, create user interfaces (Android, etc.) for making calls that are more convenient than their website tool). I don't see a business model there, but it would be really cool
You can string this stuff together with GV and other existing services, so I know it works :)