Once you have entered this information, if you set the Show When Locked option to On anyone who finds you in distress or anyone [honest] who finds your iPhone in the back of a taxi or down the side of a sofa can press Emergency in the bottom left hand corner of the unlock screen and then tap Medical ID in the same place on the following screen.  This will give them access to the emergency medical and contact details for the owner of the phone.

We are regularly visited by honest London black taxi drivers, attempting to find the owner of an iPhone, accidentally left on the back seat of their cab.  If a lost iPhone is taken into an AppleStore the staff will not reveal the registered owner of the phone to a third party due to the data protection act so the original owner cannot be traced. 

If you take our tip, it may just help a good samaritan to return your lost iPhone and in extreme circumstances, the information entered, may just save a life!