The rumor mill thinks that Necrons are going to be featured in the Shield of Baal campaign that GW is just now starting to release.
The first book in the series, Leviathan, appears to be about the Astra Militarum and Adepta Sororitas fighting against the Tyranids. Supposedly, the world on which this fight takes place is protected by the Blood Angels. The idea is that they will appear maybe in the next book. The Necrons supposedly awaken from an underground tomb and help the Blood Angles fight off the 'Nid menace. I'm not sure how that'll translate into the game rules, but if true I would expect at the least to see some Necron formations.
I'd absolutely love to see a "Canoptek" formation of some sort. It's likely that there will be a campaign box set with maybe Blood Angels vs. Necrons or Blood Angels vs. Tyranids. The rumor mill seems to be favoring Necrons right now. I am actually pretty excited to see what happens. Either way, I should be picking up a copy of Leviathan next week when it releases.