I'm sure there are misconceptions among the general public, but I can't believe EMT's and ER personnel are not trained in this simple area.
I'll put it this way-- if some ER Doctor ordered "no resuscitation" because somebody told him the patient had a Living Will, then I would love to handle that lawsuit.
For just a random taste of the serious confusion on the ground among paramedics, sample this posting and the comments on a paramedic blog:
http://urbanparamedic.blogspot.com/2007/12/confusing-mess-that-is-dnr.html