That Jon Snow should be Eddard Starks is one posibility, though not nearly the only, and not the most likely i think. Even though Grrm likes to give all his chareters both good and bad sides, i dont think eddard starks bad side is adultery. It just seems to dishourable for him... i am more into thinking his bad side is being a bit blind to everything that happens around him.

and i like the theories about Jon Snow being one of the dragons head... but who knows (Write George!! Write!!!)