I'm running with the theory (from Donny) that the prophecy was planted by Dumbledore as a plot to lure Voldemort into a trap, where Lily completed a dark ritual to protect Harry and destroy Voldemort should he attack Harry.

Objection. I don't believe Lily completed a ritual. She's not the one who spoke the words in the correct order. I don't think Voldemort would accidentally confer a protection upon an intended victim, either.

My version of this theory has Dumbledore creating the same setup as in canon because he thought it would lead to the same result as in canon. Not because he'd read the novels or anything, but because it looked like the logical outcome to him. He's brilliant, but no rationalist, and he fell prey to the conjunction fallacy, formulating a complicated plot with lots of plausible-sounding detail.

Still, he got two out of three right. Lily and Snape acted as expected. If only Voldemort had been a little dumber.

ETA, just to be clear: I think Dumbledore expected Harry to be protected and Voldemort to be defeated by the power of Lily's love, as in canon. He thinks Voldemort would be blind to an attack from this direction, because

"it is evil which does not know love, and dares not imagine love, and cannot ever understand love without ceasing to be evil."

ETA2: Actually, the part about the conjunction fallacy is wrong. I mistook Snape's involvement for an extraneous detail, but it's not. For the protective magic to activate, Lily needed to have been offered the chance to survive. Snape's role was to beg Voldemort to spare her. So every piece of the plot has a function.