Well the internets have definitely been abuzz with new Tyranids rumors over the last week or so (along with Imperial Guard, Orks, heh and more).
So who know whats going on for sure.
I can tell you though, that GW loves to release Xenos races late in the year, with every fall/winter for the last five years seeing something Xenos-ish around then.
Here’s the latest hotlinks form the tubes -MBG
Tyranid Allies? http://natfka.blogspot.com/2013/08/tyranids-can-ally-with-themselves.html
Tyranid Tidbits http://apocalypse40k.blogspot.com/2013/08/new-tyranid-rumors.html
