Well it's free to try.
No, the IPhone publishing addon is free. You have to own the engine to be able to publish anyway, and the engine is not free. I remember Taumel saying recently that Unity were charging a high price for IPhone publishing, and that was also in addition to owning an engine license.
Am I reading it right that you can't use it without having formally joined the iPhone Developer Program?
I would assume so. You can't develop for the IPhone in any shape or form without joining the program, so I can't see how the Shiva developers would have negotiated an exception to that. Particularly since it seems to deploy directly to the IPhone store.