Brad Pitt turned down the lead role of Will Freeman on the grounds that it was implausible that someone so attractive would need to pretend to be a single father to meet women. He nevertheless appears in the film (sort of) - he is on the cover of an issue of 'Esquire' magazine that Will is reading in his flat.
At the beginning of the film Marcus says he would be able to take care of his mum (Toni Collette) if he were Haley Joel Osment. Collette played Haley Joel Osment's mother in 1999 film The Sixth Sense.
As Will describes web-based research as a "unit of time" in his day, he is seen typing the URL supermodelswithseethroughtops.com. This is actually an active Web site consisting of only a picture of the Pope, Pope John Paul II, in his "Pope-mobile".