If you look closely, you'll see that the Na'vi have a little muscle running down their necks. We've got them, too—it's called the sternocleidomastoid muscle—and it's a uniquely mammalian feature. Ours make a very distinctive V-shape, and when creature designers want an alien to seem attractive and familiar to its human viewers, they often slap one on. "Even C3PO has it, in the form of little pistons on his neck. Watch Star Trek: The good guys always have them, and the bad guys don't. It's a classic alien designer trick."Actually, I wonder if they aren't actually plants, a la the Delvians of Farscape, another bunch of blue skinned, sexy aliens.
More: Mark Morford on the kinky hotness of the Na'vi.