Bluephone calls use the GSM network. When they transfer a call to use the Bluetooth link, they just transfer the first few yards of that call.
So your phone is communicating with a Bluetooth base station in your house. How is the base station communicating with the world? The Slashdot blurb says, "It doesn't roam calls onto the Internet, or even onto the landline, where they would be cheaper." What does that leave?
Is the Bluetooth base station communicating with a GSM tower over the air the same way phones do? If so, what are you gaining? Aren't you using the same amount of GSM bandwidth as before, plus some Bluetooth bandwidth?