This is a smart move by T-Mobile. It embraces the risks that "free" WiFi VoIP pose to their business. The advantages to the user (over Skype on a PDA / smartphone for instance) are significant:
- A single point-of-contact number.
- Seamless roaming in and out of the house or hotspot.
- Simple administration: simpler than buying Skype-in and Skype-out seperately and distributing an extra set of numbers to your contacts with special instructions.
The question is how do you charge for it: flat rate, per call, per second, etc... If you can get that right, you could mitigate the risk to revenue posed by WiFi VoIP and also get the customers themselves to fix local cover holes (and possibly those of their immediate neighbours).
A fascinating challenge to the traditional model of cellular planning.