from imdb:
"Borrowing a page from producers of DVD movie packages, Fox Searchlight plans to attach an alternate downbeat ending to all 1,400 prints of the apocalyptic horror film 28 Days Later, on July 25, the Los Angeles Times reported today (Wednesday). The studio said it knew no other instance in which an alternate ending had ever been added to a film while it was still in release. According to the newspaper, the ending now seen in theaters was the one that appeared in the original script, but it was ditched in favor of a grimmer conclusion after it did not test well. Director Danny Boyle told the Times that he decided, "We can't do this to people, because it was such a tough journey anyway." The $8-million film earned $9.8 million in the U.K. last year and has thus far earned $29 million in North America"

that's an idea i'd like to see more of. especially in cases where a dark original ending was cut in favor of a kitsch happy ending because test audiences didn't like it.