I see a lot of discussion, though, on what signs like that mean for owner liability. You see articles of cases in which a burglar breaks into a house, hurts himself, and then successfully sues the family. You can imagine such a thing makes a dog owner nervous. "I broke into the house and was carrying out the TV when their dog bit me! I'm suing them for a million dollars, and so that the dog gets put down!" I read things like that and think "really?" It can happen though.
Really, the question is "When is it appropriate for a dog to bite?" When the owner says so, I guess, if you want a punchline. The "real answer" seems to be when a human being is put in danger by another human. Mostly, though, it seems we prefer dogs to be visual and auditory deterrents. I've heard that a house with a dog in it is far likely to be robbed than one that does not have a dog. It makes sense, really; burglaries are frequently opportunistic crimes, the easier the better. Who wants to deal with the noise (and possibly teeth) of a dog, especially if the dog is not visible from the outside.
The laws vary state by state. I've seen a lot of discussion on whether a "Beware of Dog" sign in fact increases owner liability in the eyes of the law, by suggesting that the owner knew the dog was dangerous and just kind of let things go. On DogBiteLaw.com, under the "New York State" section, I read:
Presence of "Beware of Dog" signs standing alone are not enough to imply that dog owner knew of his dog's vicious propensities. Altmann vs Emigrant Savings Bank, 249 AD2d 67, 68 (First Dept, 1998); Frantz vs McGonagle, 242 AD2d 888 (Fourth Dept, 1997); Arcara vs Whytas, 219 AD2d 871, 872 (Fourth Dept, 1995)
So okay then. If I oped to have a "Beware of Dog", "Doberman on Duty", or "Dog on Premises" (this latter is, I think, considered to be the most neutral in terms of signage. I am not a lawyer, though, nor qualified to give legal advice. Research this on your own, please, and consult professionals), the sign alone would not be sufficient grounds to prove, in court, that my Doberman is vicious and was known to be vicious. So that's good.
Of course, you could also go completely neutral and just have a picture of your dog on the door, with a "No Trespassing" sign. I used to think that things like that were understood. If a door is closed and locked, maybe you shouldn't come in? There are reasons I'm not a lawyer, I suppose.