This was discussed without spoilers some few weeks ago (and I called it more than 2 seasons ago ;P)
"Look at the big brain on Brad!!"
Why would that be some big secret that Ned would clam Jon as his. Robert had lots of know bastards.
He doesn't have a lot bastards that had a noblewoman for a mother(that we know of) though. A noblewoman who happened to be a member of the most powerful family in the North at the time. Potentially while he was also already married to a Queen that he was "fathering" descendants with. Even if he(or rather Jamie) hadn't got a kid on Cersei when Jon was born, you think Tywin doesn't stoop to killing a kid he thinks
could usurp any future Lannister heirs(particularly since Jon would have actually been Robert's son, not Cersei's incest baby that everyone was pretending was Robert's)? He'd totally do it just to tie up loose ends.
Anyway, if it's been discussed already you can all move along. I don't check in to this thread very often because it's quite frequently an abortion.

Carry on.