I've taken a slightly different approach. I use a helmet ID tag. http://fiddledeeids.com/helmet-id-tag-p-88.html

I figure if I'm in an accident one of the first things they'll do is check my head. And if my helmet has been knocked all the way off, I'm not sure there's much they'll be able to do for me anyway. The other reason I went in this direction is that my phone is locked and can only be opened with a password (the reason for this is another story).