From particle_person: Wikipedia's comprehensive list of April Fool's website gags.
News stories I would not trust further than I could throw them:
Cloverfield Sequel To Be Animated.
Tilda Swinton to Appear in Dawn Treader; Disney to Produce Direct-to-Video Caspian Spinoff.
How 'Watchmen' Should Have Been Cast.
BREAKING: Christian Bale and Brandon Routh Sign for 'Justice League!!!' (sigh).
News stories I am tolerably sure can be trusted:
Heaven Is Wonderful For AJ Michalka In Peter Jackson’s ‘Lovely Bones’ (previous confirmation).
The Joker's Worldwide Bowling Alley Treasure Hunt (we were told there would be a Dark Knight/Joker event on 4/1).
EXCLUSIVE: Final Poster for 'Where in the World is Osama Bin Laden?' (it's a real movie).
Shekhar Kapur Helming 'Larklight' (multiple confirmations).
Brad Pitt Looking For 'Lost City of Z' (multiple confirmations).
Johnny Depp will not be appearing in condom ads. Woe.