There's a lot of talk about how Voldemort's final Horcrux was to be a person. A lot of people seem to think this was supposed to be Harry, but I don't think so, since Voldemort has mentioned clearly that he intended to kill Harry, and I don't think you'd kill your own Horcrux.
Another big topic of discussion is why was Lily given the choice to get out of the way and save herself? Rowling said in the mugglenet interview on the 16th that James had no choice, Voldemort was going to kill him anyway. I think it's possible that James was going to be the person killed in order to make Lily a horcrux. That would explain a motivation for Voldemort to let her live, and she'd be a really powerful witch to have on his side if an Imperius Curse was used.
Simplified: Voldemort went to the Potters' home to do something to all three. Harry, naturally, was to be killed. Lily was to be made into a Horcrux and possibly forced to join Voldemort, and James was the murder that would be needed to make her one. When Lily wouldn't get out of the way, though, Voldemort probably figured "well, this isn't what the plan was, but I'll kill her, kill Harry, and find someone else to be the last Horcrux."
If this is true, what are the chances that he was never able to create the seventh Horcrux? He lost his powers when the plan to kill Harry backfired, so he wasn't able to go back and do that. What if Harry is caught off guard while wasting time looking for the seventh (intended but not realized) Horcrux, and that's how the final battle begins?
Just a theory to put up with all the others, but I think it's a little more likely than that he was in love with her or she was his secret love child or something.