A rough Avatar deleted scene shows what happens after the end of the movie. And apparently some of our friendly blue Na'vis have been makin' with the Tsaheylu because somebody's preggers. Check it out!
Go ahead and fast forward 3:40 for the deleted scene.
Alright, now, it didn't technically show that it was Jake and Neytiri, but that blue template certainly had the old hair of Jake (something they would probably have changed later on) and that certainly looks like Neytiri. Besides, you know James Cameron wanted to throw in yet one more cheeseball stunt on top of the already heaping cheese platter. Why not have Neytiri pregnant — it completes the circle of life! Then again, who knew the Avatars were genetically perfect enough to knock someone up?