Dave the Mobile Warrior just informed me that T-Mobile is enabling Free Wi-Fi calls to all customers with Hotspot Calling phones.
It's not definite but if he's right, I'm gonna have to elevate him to Dave the Mobile King or something. I'm hoping this is true and will most more info as soon as we get confirmation.
As far as I know, you're suppose to pay an extra $10 a month for this. Not all phones support this. For instance, the G1 doesn't and T-Mobile has no plans to support it (though the data plan does include T-Mobile Hotspot access).
Hopefully, Skype or some other VOIP solution will come to the rescue for G1ers. For Blackberry and other UMA enabled phone users, you luck folks!