I don't have an answer but I've wondered about that, too. Ever since we adopted our Siberian Husky at age 5, I have noticed that her tongue often sticks out just a bit (like the little pink felt tongue on teddy bears). It's like she doesn't even know where her tongue is. I asked our vet once and he just answered 'sounds like she's a happy dog to me'.
I think it means that your dog is happy
I have noticed this too. My dog lucky has done this since he has been 6 years old maybe 5.(he is 6 now).I don't have a answer but a theory.I have noticed that older dogs do this and not younger.I have Lucky's son . Pudge only a year younger.I don't know like I said but I fear that it is old age.I want to know so.