I'm not sure, as I said, I just saw a mention about this the other day. Obviously, something like an Iridium is a larger handset and antenna than an iPhone. It sounds feasible using ground relay, but as far as it being standalone for use anywhere as a direct uplink, there are some technical challenges - namely output power, as you suggested. And there is not an antenna that would work for sat as best as I can tell. WOuld be cool though.