The story was "Rite of Spring" (http://www.whofic.com/viewstory.php?sid=21102&warning=Adult) -- part of my gigantic Ten/Jack slash series, "Flowers" (to give fair warning if slash isn't your thing). The reference takes place at the beginning of Chapter Two (which is safely slash-free -- that part doesn't kick in until the end of the chapter), in which Jack leads the Doctor on a game of hide-and-seek through Cardiff.