Most of the mystery has now been answered, folks. Screenwriter Scott Neustadter wrote a post for the Daily Mail called "(500) Days of Summer: Revenge is writing a film about the girl who dumped you," outlining the story behind the story. Yes, there is a girl, who may or may not be called Jenny Beckman. While they didn't work together at a greeting card company (they met at school), he did fall hard, she didn't quite feel the same way, and the pair still tried to make a casual relationship out of the deal.
The intro is a little smidge of hyperbole with its "It is surely the greatest act of revenge in the history of cinema," but it's a good read, and is nice to see how real experiences can be translated to the big screen. Even better, our "What happened next?" curiosity can be satisfied. Neustadter did show her the script, and as he says: "She loved the story, she said. It had surprised and moved her because she really related to Tom. Yes, incredibly, Jenny hadn't recognised herself as Summer at all."