Cellphones are pretty good for calling 911, or letting your family know where you are, but if you get in trouble off the grid things can go wrong in a hurry. Coverage is far from universal, especially in rural or wilderness areas.
The folks at SPOT, a satellite service company, have a handy solution that lets your iPhone connect to a global satellite network through the SPOT hardware. Along with a dedicated app, the SPOT Connect transmitter can get your distress message out, along with your exact location anywhere in the world.
Similar to the standalone SPOT Satellite GPS Messenger and the SPOT Personal Tracker (both of which work without a phone, but offer a more limited set of message options) the hardware is a small device with a GPS receiver and a satellite transmitter. The SPOT Connect pairs with your iPhone via Bluetooth; you run the SPOT app, and your iPhone suddenly becomes a satellite communicator.
The service is not built for voice, but you can update Twitter or Facebook, send short email messages or text messages (41 characters long for free input, up to 120 characters if you pre-program them), update a plot of your travels on the SPOT website for family & friends, or send your coordinates out to the GEOS International Emergency Response Coordination Center to get help on the way. This seems perfect for sailors, hikers, pilots or people exploring places where cell networks are unreliable or don't exist at all.
The device is available direct from SPOT, or at a variety of retailers like Best Buy, REI and West Marine. It lists for US $149.95, and there is a mail-in $50.00 rebate available now. The needed app is free. You'll also need a subscription that gets you the satellite time ($99.00 a year). There are some extra fees for extended services. This can literally be a lifesaver if you find yourself in the wilds, and it's a clever integration of iPhone hardware and satellite technology.