The shot of Will reflected in the mirror when he is depressed was actually taken when Hugh Grant was taking a break. The directors noticed him doing this and from behind he looked depressed, so they shot it without him knowing.
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 the novel, the music that Will introduces Marcus to - and so in turn bonds with Ellie over - is not rap, but the band Nirvana. The book is actually set during the 1993-1994 period, and the story climaxes with the kids learning of and being affected by Nirvana lead singer Kurt Cobain's 4/5/94 suicide (which within the context of the story has additional significance for Marcus). The title of the story is itself a pun on the Nirvana song "About A Girl".
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".