It made Rob's move at the end that much sweeter.
I don't have a clue who his mistress was, did you figure it out?
There are tons of theories. Some think Ned isn't Jon's father.
Ned would have been too honorable to cheat. Ned's sister, Lyanna died in a "bed of blood" (childbirth?). Jon could be the heir to Rhaegar Tarygaen. Ned claimed Jon as a bastard to keep Robert Baratheon from killing him.