Well, we found the fork.  After searching in/under/around everything, including the garbage can, Kevin and I  had given up, chalking it up to the vortex of lost things, like his putter :).  The next day, we were sitting around playing with the kids in the living room, when I saw something that struck me.  Derek was trying to shove a toy car in one of the holes of his new basketball toy.  So I turned to Kevin and said, "I wonder if he tried to stick the fork in there."  It made sense, especially since I was convinced all along that he had never left the living room with it anyway.  Well, upon close inspection, there it was.  Case solved!