I have read some theories in the past the Rheagar's son, Aegon may still be alive (though GRRM has confirmed that his daughter was definitely killed). With that theory in mind, take a look at page 493 (Hard cover version of Storm of Swords, Aria Chapter) when Aria meets "Ned" Edric Dayne. Edric is described as "..Dornishmen were small and swarthy, with black hair..., bu Ned had bigblue eyes, so dark that they looked almost purple. And his hair was pale blond, more ash than honey." Then on page 494 Ned (out of the blue) asks about Jon Snow, and calls him his "milk brother" My theory is that perhaps he is really his brother, and they are both Rhegar's sons. I don't see too much evidence for this anywhere else, but I think something important is hidden here.