In AVATAR the Na'vi are not placental mammals and breasts were for cinematic appeal. So are the Na'vi mammals at all?
The classification system we use for Earth flora and fauna can't be applied to the flora and fauna of Pandora, as they are not from the same ecosystems. Completely separate environments. So none of the Pandora creatures are mammals, as they share no common descent with any mammals.
There is no chance they have any common descent with Earth life. 'Mammal' is a term that only makes sense in the context of Earth's evolutionary tree. No animal that didn't evolve on earth (or at least from a species that originated there) can be a mammal.
It would not, however, be inaccurate to describe them as 'mammalian' due to the fact that they are endothermic, vertebrate, hair-possessing creatures who (apparently) nurse their young by way of mammary glands.
They posses many traits which mammals also posses, but are not mammals.
Edit: James Cameron himself has spoken out about the Na'vi, and has explicitly stated that they are not placental mammals. (Warning, link contains some potentially NSFW language with regards to human sexuality and anatomy).
From my memory of biology, the distinction between mammals and other chordates is that mammals have one bone in their lower mandible. Other chordates will have more bones in the lower mandible, although some may be fused into what appears to be a single bone.
When you are dealing with skeletons, there is no way to determine if the animal had hair or boobies, so one cannot use them to determine mammalian/not.