Sometime after the red Wedding but before the close of the season Ramsay is going to reveal who he is, and probably this season or very early next he'll tell Theon about the RW and his dad's involvement. Theon will say something along the lines of 'But, if you betrayed them then why torture me for doing so?' and get back a 'because I just really enjoy torturing people, haven't you been paying attention?'
As to the rest, I suspect things will go as per the books but I think there's a chance Robb's renamed, revamped wife might do the deed for Robb or Catelyn, to add to the treachery. So if she's not at the RW, things proceed as normal, but if they show her at the RW then I think she'll be complicit, and possibly killed too, maybe by Catelyn.
- Albert Einstein
King of Cairhien 20-7-2
Chancellor of the Landsraad, Archduke of Is'Mod