See, I voted "depends on the circumstance", but I've never considered myself as the kind of person who likes people names for dogs. That said, after hearing a lot of your responses, I've started thinking that there are some people names that seem as much like dog names as like people names.
From the above posters, names like Max, Sadie, Sasha, Otto, Rory, Oliver, and Daisy seem to be somewhere in the middle in my opinion. Yes, it is technically a "people name" in that there are people out there with the name, but I don't think that anyone would give you a second glance if you were to put that name on your dog. On the otherhand, a dog named "Rachel" or "Robert" might get some comments.
I might name a dog one of the aforementioned dog/people names or other "people" names that seem to straddle that line. The line is extremely arbitrary and for my purposes, I will draw it as I see fit
Some examples:
I would not be likely to name a dog Isaac or Elena or Caroline. Those to me are truly just "people names."
I might name a dog Maggie, Max, or Lucy. These to me are names that could be considered "people names" or "dog names."
I might also name a dog Boo or Buddy or Frasier. These to me are just "dog names"