When she was about 2, our 6-year-old puppy started using her tail to communicate "yes" or "no."
She barks to tell me something, and usually I know exactly what she wants from the type of bark... but in the instances I don't, she'll keep wagging her tail while I ask her various things "You want to go O-U-T?" "You want pumpkin?" "You want to call Daddy?" etc... if I've asked her about something she does NOT want, the tail will come to a freezing halt! Then when I ask her the right question, the wagging starts again (usually accompanied by a cute little bark).
We've always thought this was adorable and hadn't known of other dogs who do this. But now we have two 15-week puppies who are smart little devils. Their tails are always up and bouncy... I've only seen one of them with their tail down once (when Jackson was being harrassed by a dog in the dog park)... until today.
Dabney gave me a little bark while wagging his tail, and I asked him if he needed to go potty. His tail immediately stopped wagging and went DOWN! I was so charmed by this I had to give him kisses... and now I'm wondering if this is commonplace doggie behavior. I know the two dogs I've had in the past never did that.