The extra scene at the end with Ewan McGregor and Renée Zellweger singing a duet was filmed at the insistence of the actors. They said that with both of them having been in musicals previously (McGregor in Moulin Rouge! (2001) and Zellweger in Chicago (2002)) that it would be a sin not to.
During the dating montage, one of the scenes in the background is of a man playing bongo drums. This footage is taken from the scene in Pillow Talk (1959) when Doris Day's and Rock Hudson's characters first meet.
There are several pieces of dialogue that mention "Nazi rocket scientists." This is a reference to Operation Paper Clip, the post-World War 2 U.S. Government project that transferred the Nazi scientists who developed the V-2 rockets that bombed London to the U.S. to work on the beginnings of the U.S. Space Program, largely in Huntsville, Alabama. The most famous of these scientists, Wernher Von Braun, had been the technical director of a Nazi rocket factory that was staffed by slave labor from the nearby Mittelbau-Dora Concentration Camp. Von Braun was essential in the creation of the Redstone missile, the basis for the U.S. Missile program, and the Saturn V rocket, the basis of the Apollo program, which sent American astronauts to the moon.
Cartoon animator/MAD artist Richard Williams made a fake MAD cover involving 'Alfred E. Neuman' as Barbara holding her book. MAD commented on its cameo in the movie as a sure-fire kiss of death for a movie.