According to Robert Kirkman, whatever it was that Jenner whispered to Rick right before the survivors left the CDC will be revealed in Season 2. Kirkman says that the writing team is “very mindful of the audiences expectations”:
“There’s teasing the audience and there’s torturing the audience, there’s a fine line. We’re going to have a lot of other stuff to deal with, but we’re definitely going to be dealing with [the whisper] soon and not torturing audiences with the mystery of that too much.”
-Dane
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[...] their friends, the anticipated reveal will be a whopper all right! From Robert Kirman himself via WalkingDeadPodcast website: (don’t worry, no spoilers) “There’s teasing the audience and there’s torturing the [...]