@Thomas,
Hmmm...that's kind of awkward, though.
I do think (unless the whole damn shebang is so customary that everyone does whatever is de rigeur automatically) that a hugger has an obligation to be sensitive, as the huggee has some obligation to indicate to the hugger if they don't want to be hugged.
I don't think the onus is just on one party.
Personally, I'd find it VERY hard to SAY "Please don't hug me" to a person in a social situation. I'd more likely just do a very quick hug, and be stiff and awkward and then feel bad!
I think it's usually pretty obvious via body language if someone is not wanting a hug, and I think huggers need to respect that.
I'd hope the hugger would assess my desires as they moved in.....if I don't know someone well, and there is not an established hug protocol between us, I would normally ask if it was ok to hug, actually, or start to move in, and do a body language "is this ok?" thing before actually making contact.