Chris never makes it back to Heaven.Chris never makes it back. During the last fifteen minutes of the movie, Chris is still a Hell-vegetable, and everything that happens is in his head.
Annie didn't make it out of Hell.Most of the movie is actually a big afterlife con job. His children and mentor are trying to find the best way to help him cope with the fact that his wife is permanently lost to him, so they trick him into making a perfect copy of her in his heaven. Since they're soul mates, the copy works. The final scene isn't reincarnation. It's a realistic (as opposed to impressionist) reworking of Chris' Heaven. Maybe Inception made too big of an impression on me, though.
Chris' Son and Mentor were lying when they said no one has been brought back from Hell before.In the book going to Hell, much like everything else, is not permanent and Chris would've been reunited with Annie eventually, he's just impatient and doesn't want to see her suffer. In the movie not only do they say her fate is eternal, but that no one has been brought back before. Odd then that they have trackers who can take people to Hell and Chris' mentor tells him what he needed to do in order to bring her back, same with his son. I'd wager that people bring back loved one's from Hell all the time, but they are told that its impossible in order to test their resolve, since it obviously takes a lot of will power to go through with it and bring them back.