Not sure exactly. I just keep coming across references about prequels being eventually made out of the D&E novellas, references that made it sound like it's something in early development (perhaps because HBO has bought the rights or an option on them or something), not just the fans hoping for it. It's purely speculation on my part that perhaps HBO might use those if they need to buy themselves a year without GOT down the line so Martin can finish and publish his last book.

Mind you, they all really keep to the official answer that if they pass the published books they'll write their own ending based on an outline "without all the bells and whistles" (GRRM's expression) that Martin has provided them. I guess it's the most probable scenario, though I think the possibility Martin will by then be far along enough in the last book that they can adapt it and broadcast it on the heels of the books' release is also quite plausible (but can easily change and become unrealistic in a few years... it's why I doubt HBO really have "a plan" beside "we'll cross that bridge when we reach the river".

I've also seen references to the fact GRMM and the show runners have had WOW/ADOS-related discussions on where the TV show could/should be heading after s3.

The big mystery remains how they intend to approach ADWD and AFFC. There are two main opinions on this. Some believe they will follow Martin and keep expanding the series, and thus will adapt those two books over roughly 3 seasons (part of it will begin in s4, with s5 and s6 devoted totally to them, getting to WoW with season 7 and depending what happens in that book, we'll see after that). Others interpret hints from the producers that the series's scope won't expand much more than what's on the table by now as clues they intend to streamline massively ADWD and AFFC for the TV version, focusing on the main story lines and the already established secondary characters (Sam, Bran, Rickon, Sansa, Brienne etc.) adding new minor players as needed in those arcs, but also cutting liberally in the other arcs. The thing about ADWD and AFFC is that they don't advance terribly most of the main story lines, and even for those which do advance their "pay off" is in stand-by waiting for WOW. It won't be all that easy to keep the audience interested, especially not if more and more minor players are introduced while the main ones appear to stall or don't show up as often. Martin faced that with the books, but readers are already far more patient and dedicated than a TV audience consisting largely of people who haven't read the books. There's not really enough material to cover 2 seasons in many arcs (Jon's, Bran's, Sansa's, even to an extent Dany).

For a TV audience, and considering the "expansion" of the series didn't make unanimity among readers, it might be the wisest choice not to lose focus and get a good season and a half (part of s4 and all of season 5) out of the main story lines of ADWD and AFFC (Jon's, Dany's, Arya's, Tyrion's, Cersei's, the Iron Islands etc.) and the more minor stuff involving Rickon, Bran, Brienne, Sam etc. while throwing away most of what can be cut without altering too massively the WoW and ADOS arcs. For e.g. if what happened with Dorne was more or less a prelude to the Martells playing a larger role in WOW, then I guess it's perfectly possible they decide to skip that prelude (have this stuff happen off-screen until WOW, which might have been GRRM's original plan too) and wait until WOW for the Martells to return (after what happens with the Red Viper, I mean).

I wouldn't be surprised it's the sort of discussions they're having with GRRM concerning where they should go from here. It could well be that GRRM will suggest cuts and alterations that if done right would land them back more or less on their feet when they reach WOW/ADOS. Writers can be the most ruthless of all when adapting their own work, or advising those who are doing it.

With ASOIAF I'm more or less convinced they intend to cut whole arcs, and to invent simpler, shorter arcs for others. How they found a way to keep Theon in the plot in s3 while cutting a whole lot of ASOS material and characters is a good indicator they will focus more on the established arcs in the TV series.