I don't have a MedicAlert (or whatever they call it) acount. The thing I have around my neck should be thrown away - wrong phone number for emergency contact. Having a clearly visible MedicAlert (or maybe others) on your body, where it can be clearly seen (a bracelet, a pendant on a necklace, with a large MedicAlert logo) should be useful by providing a contact that can be used to get your medical information. I'm not sure about how useful a card in a wallet will be if a) they remove your clothes when starting to treat you and don't think to look for a wallet or b) it's soaked with urine or blood (if you printed your card with an inkjet printer, in this case, the information on the card may be smudged or unreadable).
At this point, although I didn't drink my own KoolAid, I'm thinking that a MedicAlert (or maybe another service that provides the same 24/7 coverage) item may be the best, most universally recognized, way to go.
One other option - perhaps - is an electronic device like the ones that they put into dogs so they can be identified when they're found. This device should be able to hold enough information that you can be identified, an information repository or relative can be called, and a history retrieved. Of course, this would involve getting the thing programmed and implanted, and the medical personnel identifying it and having a device to read it with. Identifying it can be easy -- get some big arrows tattooed on your arm (or wherever the thing is implanted) pointing to it (yeah, I may be joking about the tattoo).
If it wasn't for fear of viruses and other nasty things an a small USB drive worn around the neck, a USB drive with history and contact information would be useful.
Finally, I hope - is the use of a QR tag, printed, laminated, and attached to a necklace or something on your wrist. QR tags are the tags that now appear online, and sometimes on product packaging, and sometimes in items in stores (for more information, scan this). Most cellphones can read QR tags. YOUR tag can have a link to your information, and may hold enough data for some basic information without providing a link. I'll look into how much information can be put into a QR tag. EMTs and hospitals should be able to easily read the QR tag.