My wife has been preaching to me for awhile to establish an 'ICE' number on my cell phone. ICE stands for In Case of Emergency. This is a standard that emergency response units like police, paramedics, and fire have been working on promoting. The idea is, if you are ever in a situation where you are not responsive and require help, your cell phone will be checked for this ICE number and it will be called. Ideally, the person who you have listed as your ICE contact will know enough about you to help with any treatment and assistance you might need.
Cell phones have become so much more than just phones anymore, they are our cameras, how we check emails, chat, manage our calendars, keep track of our friends' phone numbers, and I've even "Twitted" once or twice from my cell phone. It's very easy for me to leave the house and forget my wallet, but rare for me to forget my cellphone nowadays.
I personally have my ICE number so it is displayed on my screen when you look at it but if you don't have that functionality or just don't care to have an ICE number displayed on you phone, then it is recommend you make an entry in your address book under, you guessed it, 'I' and make the entry ICE. It's simple and extremely helpful if ever needed.
I've actually got a friend who has taken this to another level and has published an iPhone application that brings a lot more than just an ICE contact to your phone. If you are lucky enough to own an iPhone, please check out Polkas iPhone Emergency Card for sale in the iTunes App store. You can read more about the application on Polka's Blog.