Apple controls what can be displayed on your iPhone when the screen is locked. In iOS 8, the Health app allows your medical information to be displayed without unlocking the phone. This is a valuable replacement for medical alert bracelets that folks with serious conditions have mostly avoided wearing. Because it will display allergies as well, it can be life-saving for those that are otherwise quite healthy.
Hopefully, if you are ever in a situation where you would need immediate help – in an unconscious state – the EMT on the scene will know how to find your Medical ID on the iPhone. There is nothing obvious until a normal "swipe" is done. If the phone is locked, there will be the word EMERGENCY to tap on. Next, you tap on "* MEDICAL ID". Finally, you see a display of your name, date of birth, medical conditions, medications, and an emergency contact. The latter would also be a boon for a lost phone if the person who finds it knows all about this.
All of this information is optional, and must be input by going into the Health app > Medical ID > Edit. Make sure you have "Show When Locked" turned on after you fill it out. There are additional fields like Medical Notes, Blood Type, and Organ Donor, as well as Weight and Height.
I'd like to see a "found my phone" option somewhere, so that I could be called or emailed by someone who did find my lost phone. There are other approaches to this issue, but it seems like a natural fit to include this.
I am hoping this Medical ID portion of the Health app becomes so popular that it is widely know by casual iPhone users as well as EMTs. It could be a lifesaver even for very healthy people.