Jessica Alba refused to pose for Playboy’s cover. Playboy obtained a photo from the studio for Alba’s latest movie and ran that.
From the letter to Playboy from Lavely & Singer (Alba’s attorneys) (via The Smoking Gun):

“. . . Playboy has established a known custom and practice (and an expectation with and among the public) that any woman whose photograph is featured on the cover appears in a nude or semi-nude pictorial in that month’s issue. In featuring Ms. Alba’s photograph on the cover of Playboy’s March Issue, it is clear that Playboy’s intent was to create a false belief and/or expectation among the public that Ms. Alba voluntarily appeared in the nude or semi-nude and that a revealing pictorial of her is contained in Playboy’s March Issue . . .”
There is also an unauthorized Playboy bunny on Ms. Alba’s breast.