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.
In Ali's room you can see a flag for the football (soccer) team Arsenal. Nick Hornby, author of the book on which the movie is based, is a die-hard Arsenal supporter.
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".
The quote "No man is an island" which Will jokingly attributes to Jon Bon Jovi, was actually written by John Donne in 1624, in his work "Meditation XVII".