Is Jon Snow (Game of Thrones) really dead?

He was at the end of book 5. Book 6 is due out in 2016. The TV show is moving forward into the new plotlines the author set up, so we'll have to wait and see if he's resurrected by the lady in Red.

He's like Schroedinger's Cat. Nobody knows whether he's dead. Martin could leave him dead, or could bring him back by all sorts of different ruses (vampire, or it was a double who was killed, or it was only a flesh wound [not likely when dozens of dudes were stabbing him like Julius C.], or one of the various gods [possibly including Bran, whatever he has turned into by now] decided to reanimate him, or Qyburn figured out how to reanimate him, or whatever.)

The writers and producers have stated that the fans of the show have to get used to the idea that Jon is dead and gone. In the novel the reader isn't 100% sure.

Unfortunately...

yep