The new Health app (iOS 8 only) contains a feature that will allow your phone to share medical data with first responders in the event of an accident. It’s not some high-tech Bluetooth thingy; it’s just as simple as filling in a few fields (emergency contacts, allergies, medical conditions, etc.) and flipping the bit to turn it on.
Once it’s set, the data can be accessed from your phone’s lock screen by pressing Emergency and then Medical ID.
I dunno if I’ll bother with any of the rest of the Health features, but this is a nice addition.