Jenny Beckman, the girl who is mentioned at the beginning of the movie, was a real girl who dumped one of the screenwriters, Scott Neustadter. Summer is based on this girl, and the script of the movie on their relationship.
During the flashback to Summer's job selling ice cream, Tom can be seen waiting in line.
According to the DVD commentary, one of the film's writers estimates that 75% of the film actually happened to him.
The studio never intended to include the documentary about love in this film, even going so far as to tell director Marc Webb not to film it. The audience's reaction during the first test screening convinced them otherwise.