As the comments to the article point out, it's not so easy. Carriers are complete arses to interoperate with.
This is not to say that this can't be a fantastic business - great barriers might mean that there's a mint to be made if you figure out a way around them - just don't underestimate the lack of cooperation from the carriers. That'll be your biggest showstopper.
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.
Not that I've heard, and it'd be easy to mistake the current options on the iPhone for that. They haven't launched the push service yet (they claim September so let's say January) and right now the phone has an option under "fetch new data" to poll every 15 minutes.
This is not to say that this can't be a fantastic business - great barriers might mean that there's a mint to be made if you figure out a way around them - just don't underestimate the lack of cooperation from the carriers. That'll be your biggest showstopper.