I'm testing the AJAX/JSON API outside of the Telhosting system.
What's the best way to ensure developers are happy with an API?
Simply by making an app myself using that API, of course. :)
Therefore, a new iPhone app is on the way that will let you manage your .tel directly from the iPhone. Get on the plane, and before you turn off your phone, update your .tel and tell everyone you're not available! Oh and while you're at it, use the GPS to show that you're at the airport.
At this point in the development of the app, I find the API easy to use and quite comprehensive. We made a couple of minor changes to simplify things, but otherwise I'm happy with it.