I think most people mean the deer is gutted (and no more) when they say field dressed.
Because of the requirement in many states to leave proof of sex on deer while in the field and in transit, I think most people leave the head on until they get home or to camp, in part, to comply with the game rules. I realize that the proof of sex requirement can be met at the other end of the deer too, but some states (Arkansas being one) don't allow anything more than gutting the deer in the field. Of course, in Arkansas you probably can't find a place where you're more than two miles (in most places one mile) from some type of road, so trying to get a deer in from wilderness areas is not really an issue.
Also, I don't think most people want to go to the trouble of being careful with the cape for the head mount (or mess with the head at all if they aren't having it mounted) out in the field unless they're backpacking it out.
It'll be interesting if there are other interpretations, but I've always assumed people mean just gutted when they say field dressed.
