Shane was starting to turn into a zombie before Rick killed him. That was an ironic stroke of luck, because if Rick had seen Shane turn without being killed or bit, that would be utterly hopeless for everyone and they'd all just commit suicide, end of story.
Is it conjecture on your part that Shane was turning in advance of death or did I miss something in the dialogue?
Purely speculation, I don't read spoilers or the comic or read/watch any interviews, so it's coming solely from what I see in the show. It wasn't anything in the dialogue, it was more the way Bernthal played the role (more mouth breathing and shuffling/hunching than usual) and especially the way his scenes were shot, such as Shane looking sinisterly from the woods, framed by trees, the way a zombie would be depicted.
I got the distinct impression that the director was giving us very subtle clues...and they shouldn't be more than subtle because it's way too early for that shoe to drop. Yet. I'd say S4 at the earliest, but I'll sure be watching like a hawk for more clues. For instance, the sheriff deputies who had no visible signs of death of any kind, yet had become zombies - that could have been suicide by poison, but armed deputies would be far more likely to choose to eat a bullet as their favored means of suicide.
And I just thought of a way we'll be able to tell early next season. If I were one of the survivors, my first thought upon learning I was infected would be, "does that mean I can become a zombie
at any time? And all the rest of you, too?" My mind would instantly snap right to the worst-case scenario and I don't think I'm an unusually negative person in this regards.
If none of the characters voice this fear early next season, like in the first episode, that is a big fat clue that the writers don't want us to be thinking about that. Yet.
EDIT: holy SHIT!
TWD's season finale got 9 million viewers! NBC should be so lucky to get numbers like that!