I'm a little surprised this one hasn't been archived already -- but perhaps, as one of the authors of this prank, I am biased. Then again, it appeared in a national magazine, and was subsequently covered by CBS News and the New York Times.
It's the story of Orion's Crystal, a Rubik's-cube-like toy that has the additional quality of being invisible. FAO Schwarz and other toy stores received many, many calls from people looking to get their hands on the invisible toy.
The year before the story appeared, I wrote a story for the same magazine about April Fools jokes that had appeared elsewhere. You probably know about most of them, but it might be of interest nonetheless. You can read it here: http://www.ericberlin.com/reader.html