At the Superman panel at San Diego Comic Con, it was stated on the panel that Krypto had died in the journey from Krypton to Earth. And that was the goat-like creature shown to Superman in Action Comics #2 by Lex Luthor.
However later on, I heard there was disquiet from DC over that statement. That it might not actually be true. But no one was saying anything.
Today's Action Comics #3 may give some home to Krypto lovers. Not only do you something rather Krypto-like with Jor-El on Krypton in the past, but you get this scene;
Okay, he still may be a ghost… but then could he also be a phantom? Trapped in the Phantom Zone, watching over Superman? Both alive, and dead, having to be perceived to become real? Schrödinger's dog perhaps?
Thanks to Aaron of A Comic Shop.