Okay, so. I'm going to assume that everyone has now seen Avatar, and hence I'll assume you're all up for massive spoilers, etc.
The movie we all just watched was not the movie we thought we watched. And this is not just some George "hahaha, I destroyed the hopes and dreams of a generation" Lucas screwing with the fanbase... this is lurking beneath the surface of Avatar from the beginning. The existence of the Na'vi.
The Na'vi have no common morphology with the rest of the planet. Sig even comments on it in a Youtube article. Why do they exist? How is it that they speak a recognizable language, and have genes close enough to human that it's possible to MIX IN HUMAN DNA with the Na'vi? How is it that the Na'vi have built in neural interfacing equipment that can instantly domesticate the larger animals and even predators? Wouldn't evolution make such a thing impossible?
The answer is that the Na'vi aren't a natural race. Eywa made them. They're close enough to human that the humans can communicate with them and think they look cute and cuddly (Ewya may have been slightly confused here), and alien enough that they can survive in the local environment. If that wasn't enough, Eywa provided with some elevated sudo privileges, so they could take advantage of the local fauna without Eywa being directly involved.
The how is easy. Eywa's a worldmind capable of transferring human neural networks into Na'vi clones on the second try, and it's entirely feasible to create a race and set it up with false memories. And something that's smart enough to create room temperature superconductor in bulk (what? You think Eywa runs on plants alone, when there's massively complex electromagnetic flux happening around the tree and the Na'vi just happening to be sitting on giant deposits of unobtainium?) will have no problem reading our electromagnetic communications. Eywa's on Alpha Centauri; it's been listening in since the first radio broadcasts.
The why is a bit harder to explain. Why would a worldmind play dumb?
Well, probably because it has a very good understanding of what happens to things that look like a threat to humanity. If we had any conception of what Eywa is, we'd be terrified, and we'd probably sterilize the entire planet before we even set foot on it. As it is, Eywa looks harmless. It looks beautiful. It's not exactly friendly, but it's the kind of environment that keeps humans focused on the trees instead of on the forest. Eywa can afford to watch and wait until it has to act.
It also makes a hell of a way to see human capabilities up close and personal. Eywa is well capable of doing a vacuum cleaner impression on every single EM communication on the planet. And when Eywa saw a human built Na'vi run out the container and disregard orders, it could lay bets that this was someone stupid and romantic enough to provide Eywa with more insight into the military command structure. The cute floaty things aren't accidental. They are Eywa's way of saying "hold up, this guy could be useful."
And Eywa gains massively out of it. Not only does it have a neural imprint of a woman with a massive amount of scientific knowledge, it also got to see a run through of the military. It had to expose itself to a certain extent to get the humans to back off, but you can be certain that the humans will have Eywa bacteria in their digestive tracts when they reach Earth. I wouldn't be surprised if Eywa had a very specific form of toxoplasmosis all ready to go if needed. (Indeed, one plot treatment specifically mentioned this possibility.) It doesn't have space travel down, but as long as it stays quiet, it can build up and infiltrate the military before they get to the point they know what they're dealing with.
And for those romantic souls thinking this was about Jake Sully... dude. THEY LOST. Jake and the Na'vi didn't stand a chance against the military, they used up most of what they had, and THEN, as soon as it looked like Eywa might actually lose a valuable resource, it acted. It was perfectly happy to use the Na'vi as cannon fodder if it meant it didn't have to show a card.
And I, for one, welcome our worldmind overlord. Life under Eywa would not be a bad thing for the vast majority of people, and Eywa would make better use of our technology and infrastructure than we could. Eywa keeps the reins loose for the most part, and doesn't enforce behaviour as much as "convincingly persuade." That may be the Na'vi, the charming public face presented by the unknowable, near-omniscient power that is Eywa, but even if the Na'vi are rolled up and tossed out like last week's pizza once we are no longer a threat (which is unlikely -- the Na'vi are too damn useful for offworld activity), then humanity, and Earth as a whole, can rest in peace knowing that something much smarter, tougher and biologically engineered to perfection took us out.