My LBS already carries something like this. It's a flash drive that is made into a bracelet. It is both Apple and PC compatible and supposedly doesn't require Internet access.

Some other ideas for how to wear/carry it. Would be to make a dog tag that is a flash drive that has all of that info and than wear it on a chain around one's neck or worked into one's shoe laces.

Another idea would be to create a web page with all of your information and than have a custom Q-code (I think that is the right name) that would direct EMTs/hospital personal to the web site.