The Truphone app is interesting. If you could do "paging" or something like SMS with it, it would be disruptive. Even though the iPhone doesn't allow background processes, having the push notification on your app icon would allow the user to receive a "page," find a Wifi connection, start up the app and call the person back.

I don't even think the network effects are that important. My girlfriend and I could use it to keep in touch with only each other without paying SMS fees during the day. Then we can tell another couple about it. Much the same happened with Skype and people using it to call their parents.