Let's just say, at some point while Jack was traveling with Rose and the Doctor, she brought up that she had met an alien head-in-a-jar named the Face of Boe. Jack mentions to her (and only her) that he was born in the Boeshane Peninsula and that he has also met the Face of Boe through some media-hype thing. Rose laughingly calls Jack "the Face of Boeshane" then gets it into her head to make a bet with him that he can't make the Doctor believe that he (pretty Jack) is, was, or will ever be the Face of Boe. Jack, being Jack, takes her up on the bet but before he can do anything they get transmatted to the GamesStation and... well, yeah.
However, Jack has now followed through on that bet and has won. Unfortunately, there is no way he can ever get that ten quid Rose owes him now.