I just read about this on a news site, then Snopes chimed in, so I'm going to do this to my cell phone right now.
In short, create an entry in your cellphone's address book that has the number (or numbers) of the person to call in an emergency. I.C.E. stands for In Case of Emergency.
As Snopes points out, it's not perfect, but since so many people are carrying cells now and most folks keep them with them at all times, it's a nice back-up form of identification and contact information.